IB: remove xrc_remote_srq_num from struct ib_send_wr

The field is only initialized in mlx, but never used.

If we want to add proper XRC support it should be done with a new
struct ib_xrc_wr.

This shrinks the various WR structures by another 4 bytes.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagig@mellanox.com>
Reviewed-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Tested-by: Haggai Eran <haggaie@mellanox.com>