Remove pipewire/src/keys.rs autogeneration support

The keys are not added so often to cause much trouble. But much more important is that when we are manually maintaining them, we can properly specify the Pipewire versions when they first appeared, and yes, were deprecated as well. This helps #145 to be implemented nicely. This does not prepare the codebase for a complete key removal, though, but no such event happened as of yet.