net-ipv6: remove unused IP6_ECN_clear() function

This function is unused, and furthermore it is buggy since it suffers
from the same issue that requires IP6_ECN_set_ce() to take a pointer
to the skb so that it may (in case of CHECKSUM_COMPLETE) update skb->csum

Instead of fixing it, let's just outright remove it.

Tested: builds, and 'git grep IP6_ECN_clear' comes up empty

Signed-off-by: Maciej Żenczykowski <maze@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>