[WFCORE-213] :  Clean unreferenced items from the content repository

We are using ContentReference to track contents in the content repository.
Se are tracking references to overlays.
Trying to use server group definition for domain mode.
Simplifying ContentReference
WFCORE-211: Fixing overzealous cleaning of HC content.
WFCORE-178: ContentRepositoryImpl should not call File.deleteOnExit

Jira: https://issues.jboss.org/browse/WFCORE-213
and https://issues.jboss.org/browse/WFCORE-211
and https://issues.jboss.org/browse/WFCORE-178
I believe contentRepository will never be null here, so it will never be null in DeploymentRemoveHandler.

I think this needs treatment like DeploymentAddHandler, so DeploymentRemoveHandler only gets a ref to a ContentRepository if this process is master or backupDC is true.

I *think* the same is true with fileRepository, but always passing the fileRepository there does no harm, because if the deployment is removed from the domain, it's not needed.