cmdutil: drop obsolete comment about updating to '.'

Obsolete since cf68e2649e0a (status: advertise --abort instead of
'update -C .' to abort a merge, 2018-08-09) and 5b04b6204931 (status:
advertise --abort instead of 'update -C .' to abort graft, 2018-08-09).

Differential Revision: https://phab.mercurial-scm.org/D5694

mercurial/cmdutil.py

@@ -607,11 +607,9 b' def _unshelvemsg():'
     return _helpmessage('hg unshelve --continue', 'hg unshelve --abort')
 
 def _graftmsg():
-    # tweakdefaults requires `update` to have a rev hence the `.`
     return _helpmessage('hg graft --continue', 'hg graft --abort')
 
 def _mergemsg():
-    # tweakdefaults requires `update` to have a rev hence the `.`
     return _helpmessage('hg commit', 'hg merge --abort')
 
 def _bisectmsg():