bcache: code cleanup in __bch_bucket_alloc_set()

In __bch_bucket_alloc_set() the lines after lable 'err:' indeed do
nothing useful after multiple cache devices are removed from bcache
code. This cleanup patch drops the useless code to save a bit CPU
cycles.

Signed-off-by: Coly Li <colyli@suse.de>
Link: https://lore.kernel.org/r/20240528120914.28705-4-colyli@suse.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>