appstream: Avoid adding <pkgname> to desktop files

This basically achieves nothing as of now. Earlier it used to be a workaround for quering webapps but webapps' now already are marked with 'type=webapp' in appstream. Furthermore, the epiphany plugin doesn't adopt the webapp if it has a (See commit 9d7633ec).

This commit nukes the "AddDesktopPackageName" xmlb fixup to the desktop files which in turns makes the query take into account both the metadata-providing sources for a system installed app: a) It's desktop file b) It's appdata. The desktop file is converted into a appdata-like XML representation by appstream and pushed into the xmlb. It provides metadata-info such as icons whereas the real appdata can provide information like categories, description etc. Refining the GsApp object of an installed app against these two components (a) and (b) is necessary to properly populate it's metadata.