RDMA: Remove unnecessary struct declaration

The declaration of struct ib_grh is uncessary here, because it is defined
at line 766.

Link: https://lore.kernel.org/r/20210510062843.15707-1-wanjiabing@vivo.com


Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>