tipc: remove dead code in struct tipc_topsrv

max_rcvbuf_size is no longer used since commit "414574a0

".

Signed-off-by: Zhaolong Zhang <zhangzl2013@126.com>
Acked-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>