rds: Remove unused rds_conn_error

This macro's last use was removed in commit d769ef81


("RDS: Update rds_conn_shutdown to work with rds_conn_path")
so make the macro and the __rds_conn_error function definition
and declaration disappear.

Signed-off-by: Joe Perches <joe@perches.com>
Acked-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>