ipv4: Remove unused definitions in net/ipv4/tcp_ipv4.c.

1) Remove ICMP_MIN_LENGTH, as it is unused.

2) Remove unneeded tcp_v4_send_check() declaration.

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