Update obfs4proxy to 0.0.5, remove the old go.net code.

New tag for socks5 and to change the upstream dependency.

The new tag gets:

• SOCKS5 support, for obfs4proxy's PTs, separate from #12535 (moved).
• Changed from code.google.com/p/go.net to golang.org/x/net for a dependency.

The gitian descriptor already handles pulling down the required code from the new location thanks to #15265 (moved), so all that's left is to remove the mentions of the old code and stop fetching the tarball from my people.tp.o directory, and updating the tag.