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: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NSagi Grimberg <sagig@mellanox.com>
Reviewed-by: NJason Gunthorpe <jgunthorpe@obsidianresearch.com>
Tested-by: NHaggai Eran <haggaie@mellanox.com>