selftests/bpf: Drop start_server_proto helper

Protocol can be set by __start_server() helper directly now, this makes
the heler start_server_proto() useless.

This patch drops it, and implenments start_server() using make_sockaddr()
and __start_server().

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Link: https://lore.kernel.org/r/55d8a04e0bb8240a5fda2da3e9bdffe6fc8547b2.1714014697.git.tanggeliang@kylinos.cn


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>