tipc: Remove unused domain argument from multicast send routine

Eliminates an unused argument from tipc_multicast(), now that this
routine can no longer be called by kernel-based applications.

Signed-off-by: Allan Stephens <Allan.Stephens@windriver.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>