discussion: Drop 'comment' alias
discussion: Drop 'comment' alias

lab commit fd4bc293d29c ("Add mr discussion creation") added a 'comment'
alias for the discussion command.  This alias was already in use by
the note command and should not be used.

Drop the 'comment' alias from the 'mr discussion' command.

Signed-off-by: Prarit Bhargava <fe4655370dc87a32329d591d6e8472329333dfad@redhat.com>

zaquestion/lab,zaquestion/lab