[JENKINS-17024] The SLOCCOUNT analysis Results not available in IVY builds

- SloccountDescriptor.isApplicable() doesn't consider IVY plugin. The method updated to just return true the same as other similar "publisher" plugins (e.g. Warnings).
- Dependency to Maven plugin can be removed from pom.xml, isApplicable() was the only place where it was referenced.
- Dependency to FindBugs plugin can be removed, it is nowhere referenced. There are "Remainings from the findbugs plugin have been removed" commits in Git log, this dependency was probably forgotten.
- The update was NOT fully tested since I have no IVY project. The only thing i tried is that SLOCCount publisher now appears in configuration of IVY job.