Remove outdated comment from page object

I'm doing that since
https://gitlab.com/gitlab-org/gitlab-qa/issues/154
is already closed.

qa/qa/page/mattermost/main.rb

@@ -4,11 +4,6 @@ module QA
   module Page
     module Mattermost
       class Main < Page::Base
-        ##
-        # TODO, define all selectors required by this page object
-        #
-        # See gitlab-org/gitlab-qa#154
-        #
         view 'app/views/projects/mattermosts/new.html.haml'
 
         def initialize