nvmet-rdma: Don't flush system_wq by default during remove_one

The .remove_one function is called for any ib_device removal.
In case the removed device has no reference in our driver, there
is no need to flush the system work queue.

Reviewed-by: Israel Rukshin <israelr@mellanox.com>
Signed-off-by: Max Gurtovoy <maxg@mellanox.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>