Remove banDuplicateClasses enforcer

Replaced by maven-duplicate-finder-plugin.  See
   https://github.com/zanata/zanata-parent/pull/28 and
   https://github.com/zanata/zanata-parent/commit/04a7b8d
Merge pull request #29 from zanata/remove-banDuplicateClasses

Remove banDuplicateClasses enforcer