cleanup: Remove gnome-shell-extension-prefs wrapper

After the old tool transformed into the extensions app, we added a wrapper for compatibility with extensions that spawned the command to open their preferences.

This was a long time ago.

As the ESM port happened since, we know that all extensions are newer than that change. And as spawning the old prefs tool is no longer accepted in review, no extension that is compatible with current shell versions still does it.