Simplify Flatpak support

• pv-wrap: Assume that Flatpak 1.11.x is at least 1.10.1-80-gcb47d83b

  All the features we need seem to have been merged into Flatpak git master. If people are running a git snapshot, let's assume it's the right git snapshot.

• pv-wrap: Remove environment variable gate for patched Flatpak

  Everything we need is in Flatpak git master now, so we don't need to allow for people who have patched their Flatpak with my WIP versions of the same code.

• pv-wrap: Suggest an appropriate flatpak override command

  The Steam Flatpak app can't have this property set by default until the Flatpak version used to build apps on Flathub's buildbot supports it, but we need it (either that or --device=shm, but --device=shm weakens the sandbox).

• pv-runtime: Cope with bubblewrap executable path being NULL

  At the moment we assume it's just "bwrap" when using Flatpak, but when we stop supporting the Flatpak sandbox escape code path, that will become meaningless.

• pv-wrap: Remove support for escaping from a Flatpak container

  We did this as a very early prototype, but it never fully worked, and required configuring Flatpak specially to allow Steam to escape from the sandbox (which obviously undermines Flatpak's intended sandboxing).

  Now that we've implemented the features we need in Flatpak's sub-sandboxing code, we have a better way to handle this, and can simplify the code by not trying to have it both ways.

/cc @denittis

Not tested yet.