tipc: clean up the process of link pushing packets

In original tipc_link_push_packet(), it pushes messages from protocol
message queue, retransmission queue and next_out queue. But as the two
first queues are removed, we can simplify its relevant code through
deleting tipc_link_push_queue().

Signed-off-by: Ying Xue <ying.xue@windriver.com>
Reviewed-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>