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

NB: This should not be merged until after https://github.com/zanata/zanata-client/pull/29 and https://github.com/zanata/zanata-server/pull/549.