cgroup: remove bcache_subsys_id which got added stealthily

cafe5635 ("bcache: A block layer cache") added a new cgroup
subsystem bcache_subsys without proper review and ack.  bcache_subsys
seems to use cgroup for group stats and per-group cache_mode
configuration.  This is very much the type of usage that we don't want
to allow.

Fortunately, CONFIG_CGROUP_BCACHE which enables bcache_subsys is
currently commented out, so this shouldn't have any upstream users.
Let's nip in the bud.  While at it, clarify in cgroup_subsys.h that no
new subsystem should be added without explicit acks from cgroup
maintainers.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Li Zefan <lizefan@huawei.com>
Cc: cgroups@vger.kernel.org
Cc: Kent Overstreet <kent.overstreet@gmail.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: linux-bcache@vger.kernel.org