bcache: remove a duplicate ->make_request_fn assignment

The make_request_fn pointer should only be assigned by blk_alloc_queue.
Fix a left over manual initialization.

Fixes: ff27668c ("bcache: pass the make_request methods to blk_queue_make_request")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>