net: remove net_invalid_timestamp()

No callers since v3.15.

Signed-off-by: Jakub Kicinski <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/linux/skbuff.h

···
 	return ktime_sub(ktime_get_real(), t);
 }
 
-static inline ktime_t net_invalid_timestamp(void)
-{
-	return 0;
-}
-
 static inline u8 skb_metadata_len(const struct sk_buff *skb)
 {
 	return skb_shinfo(skb)->meta_len;