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 8fa72d234da9b6b473bbb1f74d533663e4996e6b.

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>