Remove outdated code comments in MessagesXx.php files

Remove outdated code comments in MessagesXx.php files

MessagesKk_arab.php:
MessagesKk_cyrl.php:
MessagesKk_latn.php:
MessagesRu.php:
* Translations of a comment warning end-users against editing
  localisation messages in these files. That comment was removed
  from  MessagesEn.php in b38247894422fb34dbddaea23cfc08cd72f48d5b,
  as we no longer store localisation messages here.

MessagesKsh.php
* Warning for translators not to use encodings other than UTF-8.

Change-Id: I1ff2cf035b84dc53c368ad7182f294c7fb8f1a3d