tcp: Remove dead code and fields for bhash2.

Now all sockets including TIME_WAIT are linked to bhash2 using
sock_common.skc_bind_node.

We no longer use inet_bind2_bucket.deathrow, sock.sk_bind2_node,
and inet_timewait_sock.tw_bind2_node.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>