temp/gnome-software: sync upstream and remove pmOS AppStream data

AppStream data for postmarketOS has been removed until the new format without a double "--" is deployed. Otherwise transition will be a pain and we would have to take care of it downstream. In the end, not having AppStream for pmOS has very little real impact since:

• There are very few apps, out of which only postmarketos-tweaks is relevant and pmOS-only
• The appstream data hasn't been generated since June
• pmos-only apps will still show properly if installed. They will simply not be discoverable if not installed.

Relates #1384 (closed)

Draft until:

• Tested
• Latest merge for GNOME Software is built. There seems to be some issue in pmbootstrap, where it can't deal with it: http://build.postmarketos.org/#queued