This reverts commit fe2fa9c6.

See failure log below (also at
https://gist.github.com/kohsuke/9dcff6789ce5eda88b4635a34e986e61)

IIUC, JENKINS-49905 was meant to test the signature of remoting, but the
change is testing the signature of core jar, which is not meant to be
signed. So this failure makes sense.

Reverting the change to proceed with the release.

----------------------
Sun Aug  5 10:49:34 PDT 2018: [INFO] [INFO] --- maven-jar-plugin:3.0.2:jar (default-jar) @ jenkins-core ---
Sun Aug  5 10:49:34 PDT 2018: [INFO] [INFO] Building jar: /files/kohsuke/ws/jenkins/rc/core/target/jenkins-core-2.136.jar
Sun Aug  5 10:49:35 PDT 2018: [INFO] [INFO]
Sun Aug  5 10:49:35 PDT 2018: [INFO] [INFO] --- maven-hpi-plugin:2.3:record-core-location (default) @ jenkins-core ---
Sun Aug  5 10:49:35 PDT 2018: [INFO] [INFO]
Sun Aug  5 10:49:35 PDT 2018: [INFO] [INFO] >>> findbugs-maven-plugin:3.0.5:check (findbugs) > :findbugs @ jenkins-core >>>
Sun Aug  5 10:49:35 PDT 2018: [INFO] [INFO]
Sun Aug  5 10:49:35 PDT 2018: [INFO] [INFO] --- findbugs-maven-plugin:3.0.5:findbugs (findbugs) @ jenkins-core ---
Sun Aug  5 10:49:35 PDT 2018: [INFO] [INFO] Fork Value is true
Sun Aug  5 10:50:10 PDT 2018: [INFO] [INFO] Done FindBugs Analysis....
Sun Aug  5 10:50:11 PDT 2018: [INFO] [INFO]
Sun Aug  5 10:50:11 PDT 2018: [INFO] [INFO] <<< findbugs-maven-plugin:3.0.5:check (findbugs) < :findbugs @ jenkins-core <<<
Sun Aug  5 10:50:11 PDT 2018: [INFO] [INFO]
Sun Aug  5 10:50:11 PDT 2018: [INFO] [INFO]
Sun Aug  5 10:50:11 PDT 2018: [INFO] [INFO] --- findbugs-maven-plugin:3.0.5:check (findbugs) @ jenkins-core ---
Sun Aug  5 10:50:11 PDT 2018: [INFO] [INFO] BugInstance size is 0
Sun Aug  5 10:50:11 PDT 2018: [INFO] [INFO] Error size is 0
Sun Aug  5 10:50:11 PDT 2018: [INFO] [INFO] No errors/warnings found
Sun Aug  5 10:50:11 PDT 2018: [INFO] [INFO]
Sun Aug  5 10:50:11 PDT 2018: [INFO] [INFO] --- maven-gpg-plugin:1.6:sign (sign-artifacts) @ jenkins-core ---
Sun Aug  5 10:50:11 PDT 2018: [INFO] [INFO]
Sun Aug  5 10:50:11 PDT 2018: [INFO] [INFO] --- maven-jarsigner-plugin:1.2:verify (verify-signature) @ jenkins-core ---
Sun Aug  5 10:50:12 PDT 2018: [INFO] [INFO] ------------------------------------------------------------------------
Sun Aug  5 10:50:12 PDT 2018: [INFO] [INFO] Reactor Summary:
Sun Aug  5 10:50:12 PDT 2018: [INFO] [INFO]
Sun Aug  5 10:50:12 PDT 2018: [INFO] [INFO] Jenkins main module 2.136 .......................... SUCCESS [  9.657 s]
Sun Aug  5 10:50:12 PDT 2018: [INFO] [INFO] Jenkins cli ........................................ SUCCESS [ 13.659 s]
Sun Aug  5 10:50:12 PDT 2018: [INFO] [INFO] Jenkins core ....................................... FAILURE [01:40 min]
Sun Aug  5 10:50:12 PDT 2018: [INFO] [INFO] Jenkins war ........................................ SKIPPED
Sun Aug  5 10:50:12 PDT 2018: [INFO] [INFO] Tests for Jenkins core 2.136 ....................... SKIPPED
Sun Aug  5 10:50:12 PDT 2018: [INFO] [INFO] ------------------------------------------------------------------------
Sun Aug  5 10:50:12 PDT 2018: [INFO] [INFO] BUILD FAILURE
Sun Aug  5 10:50:12 PDT 2018: [INFO] [INFO] ------------------------------------------------------------------------
Sun Aug  5 10:50:12 PDT 2018: [INFO] [INFO] Total time: 02:04 min
Sun Aug  5 10:50:12 PDT 2018: [INFO] [INFO] Finished at: 2018-08-05T10:50:12-07:00
Sun Aug  5 10:50:12 PDT 2018: [INFO] [INFO] ------------------------------------------------------------------------
Sun Aug  5 10:50:12 PDT 2018: [INFO] [ERROR] Failed to execute goal org.apache.maven.plugins:maven-jarsigner-plugin:1.2:verify (verify-signature) on project jenkins-core: Failed executing '/bin/sh -c cd /files/kohsuke/ws/jenkins/rc/core && /usr/local/jdk1.8.0_144/jre/../bin/jarsigner -verify -strict -certs /files/kohsuke/ws/jenkins/rc/core/target/jenkins-core-2.136.jar' - exitcode 16 -> [Help 1]
Sun Aug  5 10:50:12 PDT 2018: [INFO] [ERROR]
Sun Aug  5 10:50:12 PDT 2018: [INFO] [ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
Sun Aug  5 10:50:12 PDT 2018: [INFO] [ERROR] Re-run Maven using the -X switch to enable full debug logging.
Sun Aug  5 10:50:12 PDT 2018: [INFO] [ERROR]
Sun Aug  5 10:50:12 PDT 2018: [INFO] [ERROR] For more information about the errors and possible solutions, please read the following articles:
Sun Aug  5 10:50:12 PDT 2018: [INFO] [ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/MojoExecutionException
Sun Aug  5 10:50:12 PDT 2018: [INFO] [ERROR]
Sun Aug  5 10:50:12 PDT 2018: [INFO] [ERROR] After correcting the problems, you can resume the build with the command
Sun Aug  5 10:50:12 PDT 2018: [INFO] [ERROR]   mvn <goals> -rf :jenkins-core
----------------------

Executing the offending command from the terminal:
----------------------
$ /bin/sh -c 'cd /files/kohsuke/ws/jenkins/rc/core && /usr/local/jdk1.8.0_144/jre/../bin/jarsigner -verify -strict -certs /files/kohsuke/ws/jenkins/rc/core/target/jenkins-core-2.136.jar'

jar is unsigned.
$ echo $?
16
----------------------

* verify-signature in core/pom.xml

* removed maven comment + add-tsa task

* Bump up jna.jar version to 4.5.1

Fixes https://issues.jenkins-ci.org/browse/JENKINS-52771.
JNA support for s390x is missing in 4.2.1. The support is added in 4.5.1 version as part of issue 845
Issue: https://github.com/java-native-access/jna/issues/845
Commit : https://github.com/matthiasblaesing/jna/commit/1fef4a20a031d91dce6d37e6ef809c9d38708bd2

* Per review comment , bump jna.jar version to 4.5.2