Do not invalidate OS upgrades in the Updates page
The Updates page resets (1) 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. Besides being a layering violation (the PkgKit plugin should be one dealing with package removals and how they affect upgrades), this can be frustrating to users, e.g. a user goes to an internet café, downloads the OS upgrade, then goes home where they're offline -> the OS upgrade will show up as needing to be downloaded again...
Thus, I propose that we remove this code and move the responsibility of dealing with incomplete OS upgrades to the plugins.
(1) https://gitlab.gnome.org/GNOME/gnome-software/blob/master/src/gs-updates-page.c#L1703