mm/memcg: remove incorrect comment

Swapcache readahead pages are charged before being used, so it is unlikely
that they will be migrated before charging.  Remove the incorrect comment.

Link: https://lkml.kernel.org/r/1605864930-49405-1-git-send-email-alex.shi@linux.alibaba.com
Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>