IB/mlx5: Eliminate GSI RX QP's send buffers

Now that the transmission of GSI MADs is done with the special transmission
QPs, eliminate the send buffers in the GSI receive QP.

Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Haggai Eran <haggaie@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>