nvme-mpath: remove I/O polling support

The ->poll_fn has been stale for a while, as a lot of places check for mq
ops.  But there is no real point in it anyway, as we don't even use
the multipath code for subsystems without multiple ports, which is usually
what we do high performance I/O to.  If it really becomes an issue we
should rework the nvme code to also skip the multipath code for any
private namespace, even if that could mean some trouble when rescanning.

Reviewed-by: Keith Busch <keith.busch@intel.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>