Stop supporting STEAM_COMPAT_LAUNCHER_SERVICE=1391110, etc.

The original design for injecting commands into container code was that it was going to be based on Steam app-IDs, but that's awkward for the compat tool authors (particularly Proton, which has a lot of branches with their own app-IDs), so the plan has changed.

It is now meant to be based on the compatmanager_layer_name in toolmanifest.vdf, which only needs to be unique within a stack that makes sense to use together: for instance, SteamLinuxRuntime_anything can share "container-runtime", because it only makes sense for any given stack to include one of them. Similarly, all Proton versions can share "proton". We already recognised those names as a developer convenience, even when they were not going to be used in production, so nothing more needs to be done to support them.

This partially reverts commit ee4b6c47 "Add app-ID-based triggering for steam-runtime-launcher-service". The addition of the --steam-app-id option has not been reverted, for backwards-compat, but it no longer does anything.