udp: Remove unused function declaration udp_bpf_get_proto()

commit 8a59f9d1 ("sock: Introduce sk->sk_prot->psock_update_sk_prot()")
left behind this.

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/20230801133902.3660-1-yuehaibing@huawei.com


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