tipc: remove unused tipc_group_size

After commit eb929a91 ("tipc: improve poll() for group member socket"),
it is no longer used.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>