bcache: remove "XXX:" comment line from run_cache_set()

In previous bcache patches for Linux v5.2, the failure code path of
run_cache_set() is tested and fixed. So now the following comment
line can be removed from run_cache_set(),
	/* XXX: test this, it's broken */

Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>