WIP: Do not invalidate OS upgrades in the Updates page

The Updates page resets the OS upgrades state to be available again if there's a change to the updates, or an app removal, etc.. The reason for this is that package-based systems need to make sure they have all the packages downloaded, so this would force the user to download any missing packages.

As a result, if the user has a downloaded OS upgrade in an ostree system, and restarts GNOME Software, or removes an app, in this state, the OS upgrade will show the Download button again, even if offline.

Any OS upgrade invalidation should be done by the plugins related to it and not by the Updates page, so this patch removes the mentioned code but the PR must NOT be merged until we have a way to invalidate pkgkit-based OS upgrades (WIP by @hughsie ).

(Issue in #384 )