As we are going to implement tproxy hijack via bpf_sk_assign, tproxy
response won't reach wan iface at all, unless wan iface == lan iface.

The only remaining "tproxy_response" is the place returning TC_ACT_PIPE
to hand packets over from tproxy_wan_egress to tproxy_lan_egress.

This commit also deletes rev-NAT logic for tproxy response.

This commit tries to make a minimum change, otherwise file diff is
too confusing to reviewers. I'll clean it up in the next patch.

This commit merely removes the `if (false)` branch at:

```
if (false) {
  // Comments
} else {
  ...
}
```

The file diff becomes completely messed up, so I split it into a
separate patch without any functional change.

Note the necessity of separation of `assign_socket_tcp` and
`assign_socket_udp`:

As `struct bpf_sock *` has different verifier types for tcp and udp, the
code below can't pass verifier:

```
static __always_inline int
assign_socket(struct __sk_buff *skb, struct bpf_sock_tuple *tuple, __u32 len,
	      __u8 nexthdr) {
	struct bpf_sock *sk;
	switch (nexthdr) {
	case IPPROTO_TCP:
		sk = bpf_sk_lookup_tcp(skb, tuple, len, BPF_F_CURRENT_NETNS, 0);
	case IPPROTO_UDP:
		sk = bpf_sk_lookup_udp(skb, tuple, len, BPF_F_CURRENT_NETNS, 0);
	}
	if (!sk) {
		return -1;
	}

	int res = bpf_sk_assign(skb, sk, 0);
	bpf_sk_release(sk);
	return res;
}
```

We no longer need tcp_dst_map for NAT. Relevant Golang logic is also
removed.

One thing need to mention is "dst_routing_result" struct. Although
tcp_dst_map is gone, dst_routing_result struct is still in use under
userspace at https://github.com/daeuniverse/dae/blob/cab1e4290967340923d7d5ca52b80f781711c18e/control/udp.go#L69C17-L69C17.
Therefore, this commit remains this struct and make some efforts to
ensure bpf objects are compiled with it.

Previously, wan_egress has to encap UDP packets with routing info, but
it's no more necessary as we are in favor of bpf_sk_assign without NAT.

This is a must-have, otherwise packets being bpf_sk_assigned and routed
to local on wan will be dropped by kernel during fib_lookup:

```
// https://github.com/torvalds/linux/blob/v6.5/net/ipv4/fib_frontend.c#L381
static int __fib_validate_source()
...
	if (res.type != RTN_UNICAST &&
	    (res.type != RTN_LOCAL || !IN_DEV_ACCEPT_LOCAL(idev)))
		goto e_inval;
...
```