btrfs: Drop unused function btrfs_abort_devices()

1) This function is not used anywhere.

2) Using the blk_abort_queue() to abort the queue seems not correct.
blk_abort_queue() is used for timeout handling (block/blk-timeout.c).

Cc: Chris Mason <chris.mason@oracle.com>
Cc: linux-btrfs@vger.kernel.org
Cc: Jens Axboe <axboe@kernel.dk>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Asias He <asias@redhat.com>