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.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>