RDS: Remove stale function rds_send_get_message()

The only caller of rds_send_get_message() was
rds_iw_send_cq_comp_handler() which was removed as part of
commit dcdede04

 ("RDS: Drop stale iWARP RDMA transport"),
so remove rds_send_get_message() for the same reason.

Signed-off-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>