ipv6: tcp: get rid of tcp_v6_clear_sk()

Now RCU lookups of IPv6 TCP sockets no longer dereference pinet6,
we do not need tcp_v6_clear_sk() anymore.

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