This reverts commit d45d18ed.

I found that literate-api library wants to use a newer version of Guava.
Sine all the usage of Guava in plugins end up picking up the version
from core, we need to rev this pretty aggressively.

Shaves >11min off the test time for maven-plugin on my machine!

See https://github.com/jenkinsci/maven-plugin

- set findbugs version to 2.5.2 to avoid:
[WARNING] 'reporting.plugins.plugin.version' for
org.codehaus.mojo:findbugs-maven-plugin is missing. ...

- delete maven-pmd-plugin and maven-checkstyle-plugin (commented out
since more than two years)