bpf, udp: Remove some pointless comments

These comments in udp_bpf_update_proto() are copied from the
original TCP code and apparently do not apply to UDP. Just
remove them.

Reported-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Cong Wang <cong.wang@bytedance.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20210403052715.13854-1-xiyou.wangcong@gmail.com