tcp: remove dead code in __tcp_v6_send_check()

For some reason, I forgot to change __tcp_v6_send_check() at
the same time I removed (ip_summed == CHECKSUM_PARTIAL) check
in __tcp_v4_send_check()

Fixes: 98be9b12 ("tcp: remove dead code after CHECKSUM_PARTIAL adoption")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>