[JENKINS-32493] Remove plugin parent POM from core project.

As part of the process of getting a [new plugin parent POM](https://github.com/jenkinsci/plugin-pom) decouple from the core project
the plugin module is removed.

pom.xml

@@ -52,7 +52,6 @@ THE SOFTWARE.
     <module>war</module>
     <module>test</module>
     <module>cli</module>
-    <module>plugins</module>
   </modules>
 
   <scm>