Taskcluster: remove flatpak build and toolchain tasks

[oskirby]

Description

In the last update to flatpaks, we revised the build jobs in Github Workflows to make better use of the official Flatpak build tooling, and eliminated the need to manually vendor the rust crates by making use of the flatpak-cargo-generator.py script.

Once 2.24 is released, my plan is to attempt to create an official release on Flathub by migrating all of that work into the flathub organization so that they can begin generating and publishing official builds for us.

As such, we no longer really have a need to perform our own Flatpak builds using taskcluster as the release pipeline for the VPN on Flathub isn't going to touch any Mozilla infrastructure at all. Therefore, I think it is time that we remove the taskcluster flatpak build and toolchains tasks.

It is not without a little bit of sadness too, because I thought the podman-worker.py was a really neat piece of work to grant privileges to Docker containers that otherwise wouldn't have been possible via the Docker workers.

Reference

i.e Jira or Github issue URL

Checklist

• My code follows the style guidelines for this project
• I have not added any packages that contain high risk or unknown licenses (GPL, LGPL, MPL, etc. consult with DevOps if in question)
• I have performed a self review of my own code
• I have commented my code PARTICULARLY in hard to understand areas
• I have added thorough tests where needed

[mcleinman]

🫗 Pour one out for the podman worker.

[strseb]

🫡