mm: memcg: fix obsolete code comments

This patch fixes/removes some obsolete comments in the code related
to the kernel memory accounting:

 - kmem_cache->memcg_params.memcg_caches has been removed by commit
   9855609b ("mm: memcg/slab: use a single set of kmem_caches for
   all accounted allocations")

 - memcg->kmemcg_id is not used as a gate for kmem accounting since
   commit 0b8f73e1

 ("mm: memcontrol: clean up alloc, online,
   offline, free functions")

Link: https://lkml.kernel.org/r/20201110184615.311974-1-guro@fb.com
Signed-off-by: Roman Gushchin <guro@fb.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>