mm/memcontrol.c: fix a -Wunused-function warning

mem_cgroup_id_get() was introduced in commit 73f576c0 ("mm:memcontrol:
fix cgroup creation failure after many small jobs").

Later, it no longer has any user since the commits,

1f47b61f ("mm: memcontrol: fix swap counter leak on swapout from offline cgroup")
58fa2a55

 ("mm: memcontrol: add sanity checks for memcg->id.ref on get/put")

so safe to remove it.

Link: http://lkml.kernel.org/r/1568648453-5482-1-git-send-email-cai@lca.pw
Signed-off-by: Qian Cai <cai@lca.pw>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.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>