Merge branch 'jk-epic-changes-ce-port' into 'master'

Remove including ee Epics module

See merge request gitlab-org/gitlab-ce!15162

app/models/epic.rb

 # Placeholder class for model that is implemented in EE
 # It will reserve (ee#3853) '&' as a reference prefix, but the table does not exists in CE
 class Epic < ActiveRecord::Base
-  prepend EE::Epic
-
   # TODO: this will be implemented as part of #3853
   def to_reference
   end