Revert "bdi: add a user-tunable cpu_list for the bdi flusher threads"

Revert "bdi: add a user-tunable cpu_list for the bdi flusher threads"

This reverts commit 4e122a0229ed90e4f144ee7febd0a8c9a607bc80.

People disagree about how this should be done, so let's revert this for
now so that nobody starts using the new tuning interface.  Tejun is
thinking about a more generic interface for thread pool affinity.

Requested-by: Tejun Heo <tj@kernel.org>
Acked-by: Jeff Moyer <jmoyer@redhat.com>
Acked-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>