block: don't clear REQ_ALLOC_CACHE for non-polled requests

Since commit:

b99182c5

 ("bio: add pcpu caching for non-polling bio_put")

we support bio caching for IRQ based IO as well, hence there's no need
to manually clear REQ_ALLOC_CACHE if we disable polling on a request.

Reviewed-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>