SLING-3286 - Remove plugin version overrides from poms

Remove most plugin version overrides. Still left in:

- a couple of projects where upgrading to the maven-bundle-plugins
  leads to errors (TODOs added)
- a couple of projects where the plugin version is newer than the
  one from the parent project ( comments added )

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1551060 13f79535-47bb-0310-9956-ffa450edef68