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.
Commit originally by @jrocha; I just tidied it up and tweaked the commit message.