flatpak: Fix launching Flatpak apps when runtime versions change

The apps rely on the runtimes declaration in the repo's AppStream file.
This information however relates to the app's version that is given by
the server, which may need a different runtime (because of a update)
than what is installed; thus, these changes drop the check that verifies
whether the runtime is installed before launching the app, delegating
that responsibility to Flatpak itself which prevents mistakenly assuming
that the runtime is not installed.

This is a partial revert of commit 6ce59bab from
https://bugzilla.gnome.org/show_bug.cgi?id=765055. Since then, flatpak
transactions mean we’re a lot better at ensuring the correct runtime is
installed when an app is installed, so the situation from the original
bug report should never really be hit any more.
1 job for stop-checking-flatpak-runtimes in 24 minutes and 17 seconds (queued for 1 second)
Status Job ID Name Coverage
passed #330841