net/rds: Remove two WARN_ON() statements

Remove two WARN_ON() statements that verify something that is guaranteed
by the RDMA API, namely that the failed_wr pointer is not touched if an
ib_post_send() call succeeds and that it points at the failed wr if an
ib_post_send() call fails.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>