RDS: TCP: Remove unused constant

RDS_TCP_DEFAULT_BUFSIZE has been unused since commit 1edd6a14
("RDS-TCP: Do not bloat sndbuf/rcvbuf in rds_tcp_tune").

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