plugin loader: Remove unavailable app converting

I don't understand what this is supposed to do. Richard says that it's supposed to kick in when searching for 'mp3 format' in gnome-software, but it doesn't seem to. Looks like unused code path to me.

For codec searches coming from other applications, we have the Extras page that handles unavailable codecs. I don't think we need to duplicate the functionality in the plugin loader.

Also, commit f32e77d3 slightly changed the translatable strings; let's just remove those before translators go and do all the hard translating work for something that's actually not used.