Don't try to add Fedora's OCI Flatpak repository on RHEL
It wasn't actually getting added because redhat-release and centos-release don't include flatpak-add-fedora-repos.service among the enabled systemd presets. Therefore, this doesn't actually change anything in the default user experience.
Moreover, this was never intended to be there for RHEL, and is merely a leftover from Fedora.
Resolves: #2051697