clatd eBPF - simplify TCP/UDP logic

We rely on checksum neutrality of chosen IPv6 address anyway,
so there really is no point to trying to pretend that we care
about the contents of the TCP/UDP headers.

This will make it easier to handle ip fragments.

Test: no-op, dead code removal, builds
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I1ed690bd48f0b8adef89f46a9fe9ba24c3a966c9