Implement RemoveCommand in maven-scm-provider-jgit with TCK test for all providers

The git scm plugins differs in support for the scm command remove. This breaks the goal prepare-with-pom of the maven-release-plugin using the maven-scm-provider-jgit. The method is not fully implemented and throws an UnsupportedOperationException. 

Referring to the "Maven SCM Providers Matrix" this feature is documented as supported and should be implemented in the jgit provider like in maven-scm-provider-gitexe.
 

Variant	remove supported	Source
maven-scm-provider-gitexe		Github - GitExeScmProvider.java - getRemoveCommand()
maven-scm-provider-jgit		Github - JGitScmProvider.java - getRemoveCommand()