ipv4: Remove unused declaration from include/net/tcp.h.

- The tcp_unhash() method in /include/net/tcp.h is no more needed, as the
unhash method in tcp_prot structure is now inet_unhash (instead of
tcp_unhash in the
past); see tcp_prot structure in net/ipv4/tcp_ipv4.c.

- So, this patch removes tcp_unhash() declaration from include/net/tcp.h

Signed-off-by: Rami Rosen <ramirose@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>