[CI] Do not re-implement the whole Flatpak job

It is for several reasons not a good practice to use custom jobs if a KDE template exist. The better solution is to only modify the parts of the template that need modification. This avoids that things go out of sync like now with recently added signing and publishing of flatpaks.

Please note that the https://labplot.kde.org/download/ needs a change ones this is merged (the Flatpak Binary Factory job is deprecated now and will soon be removed):

Install Flatpak using

flatpak install kdeapps https://cdn.kde.org/flatpak/org.kde.labplot2.flatpakref

See also https://userbase.kde.org/Tutorials/Flatpak#Nightly_KDE_Apps