Remove unused message 'postcomment'

* No longer used in extensions/InputBox as of 6cfaec4f1ac8.
* No longer used in skins/Nostalgia as of 76cfc18b4e38 (actually
  earlier because LegacyTemplate::commentLink() was not called anyway).

Change-Id: I648cfc8ccb5396db2a4c836d9888f18133d432fb
Merge "Remove unused message 'postcomment'"