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 4def0da6f5cdd6554da93a17d2b447fee79c2a53.

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>