Clean up app/controllers/projects/merge_requests_controller.rb after MR refactor

We have a ToDo in merge_requests_controller.rbre: cleaning up after the MR Refactor.

def show
...
        # TODO cleanup- Fatih Simon Create an issue to remove these after the refactoring
        # we no longer render notes here. I see it will require a small frontend refactoring,
        # since we gather some data from this collection.
        @discussions = @merge_request.discussions
        @notes = prepare_notes_for_rendering(@discussions.flat_map(&:notes), @noteable)
...
end

in https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/controllers/projects/merge_requests_controller.rb#L45