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>

include/rdma/ib_verbs.h

@@ -2139,7 +2139,6 @@ struct ib_flow_action {
 };
 
 struct ib_mad;
-struct ib_grh;
 
 enum ib_process_mad_flags {
 	IB_MAD_IGNORE_MKEY	= 1,