gs-repos-dialog: Cannot disable all 3rd-party repositories

Merged Milan Crha requested to merge wip/mcrha/3rd-party-repo-disable into main

All the 3rd-party repositories should be disable-able, thus the 3rd-party section should not use the heuristics to disallow disable of some of them. This could be seen on a 'flathub' Flatpak repository installed for the system, which is not allowed to be disabled in the Flatpak section.

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