Revert "block/mq-deadline: Add cgroup support"

This reverts commit 08a9ad8b ("block/mq-deadline: Add cgroup support")
and a follow-up commit c06bc5a3 ("block/mq-deadline: Remove a
WARN_ON_ONCE() call"). The added cgroup support has the following issues:

* It breaks cgroup interface file format rule by adding custom elements to a
  nested key-value file.

* It registers mq-deadline as a cgroup-aware policy even though all it's
  doing is collecting per-cgroup stats. Even if we need these stats, this
  isn't the right way to add them.

* It hasn't been reviewed from cgroup side.

Cc: Bart Van Assche <bvanassche@acm.org>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>