RDMA/usnic: Remove the query_pkey callback

Now that the query_pkey() isn't mandatory by the RDMA core, this callback
can be removed from the usnic provider. The libfabric userspace never
touches the pkey.

Link: https://lore.kernel.org/r/20200820125346.111902-1-kamalheib1@gmail.com


Signed-off-by: Kamal Heib <kamalheib1@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>