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>

include/net/inet_ecn.h

@@ -133,11 +133,6 @@ static inline int IP6_ECN_set_ce(struct sk_buff *skb, struct ipv6hdr *iph)
 	return 1;
 }
 
-static inline void IP6_ECN_clear(struct ipv6hdr *iph)
-{
-	*(__be32*)iph &= ~htonl(INET_ECN_MASK << 20);
-}
-
 static inline void ipv6_copy_dscp(unsigned int dscp, struct ipv6hdr *inner)
 {
 	dscp &= ~INET_ECN_MASK;