[FIXED JENKINS-19332]

- 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)
Merge pull request #917 from mvach/JENKINS-19332

[FIXED JENKINS-19332]