blk-mq-rdma: remove queue mapping helper for rdma devices

No rdma device exposes its irq vectors affinity today. So the only
mapping that we have left, is the default blk_mq_map_queues, which
we fallback to anyways. Also fixup the only consumer of this helper
(nvme-rdma).

Remove this now dead code.

Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Acked-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>