inet6: Remove unused function declaration udpv6_connect()

This is never implemented since the beginning of git history.
Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://lore.kernel.org/r/20230731140437.37056-1-yuehaibing@huawei.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/transp_v6.h

@@ -33,8 +33,6 @@ void udplitev6_exit(void);
 int tcpv6_init(void);
 void tcpv6_exit(void);
 
-int udpv6_connect(struct sock *sk, struct sockaddr *uaddr, int addr_len);
-
 /* this does all the common and the specific ctl work */
 void ip6_datagram_recv_ctl(struct sock *sk, struct msghdr *msg,
 			   struct sk_buff *skb);