backend: Remove support for installing codecs on-demand
The only way to distribute totem we support is through Flathub's Flatpak, and this process isn't needed as Flathub ships all the necessary codecs.
Seeing as totem didn't have any control over the installation of plugins, its UI or workflow, the process was always very dependent on end-users having prior knowledge of where to get codecs, or they would end up installing only codecs for either audio or video, and wondering how this would all work.