quic: drop support for QUIC draft-29

[MarcoPolo]

This does the minimal amount of changes to drop draft29 without updating quic-go.

I'm thinking, for now at least, that we'll leave machinery of the multiversion QUIC for a couple of reasons:

1. It'll make adding support for QUIC-v2 very easy.
   a. This would mostly be greasing exercise to avoid ossification on quic-v1. There's nothing QUIC-v2 gives us that we don't have with QUIC-v1.
2. It'll avoid us from reinventing this if we re-introduce this. This is tricky code and this code works and has been deployed for a bit.
3. It's stable as is, and a refactor would introduce more variables to the next release (already quite big with quic-go changes).

[marten-seemann]

Thank you for splitting up the PR @MarcoPolo!