Nuke `dns_spoofing.py` addon and fix docs

I think we do all of this automatically now.

1. Verify that mitmproxy works properly in regular mode if traffic arrives with DNS spoofing.
2. Update the proxy mode docs to reflect that.
3. Nuke the dns_spoofing.py addon. It probably makes sense to keep a placeholder file for a while pointing at the updated docs.

The big chunk of work is likely in 2. If anyone can confirm 1., that would be awesome!