trivial: Remove any support for installed appdata-extra files
trivial: Remove any support for installed appdata-extra files

ikeydoherty/appstream-glib,hughsie/appstream-glib,ximion/appstream-glib,ximion/appstream-glib,hughsie/appstream-glib,mitya57/appstream-glib,mitya57/appstream-glib,ikeydoherty/appstream-glib,superm1/appstream-glib,superm1/appstream-glib,hughsie/appstream-glib,ximion/appstream-glib