Do not depend on `--unified-core`

[travier]

Unfortunately, neither Silverblue/Kinoite or IoT use unified-core mode in rpm-ostree yet. It's planned but we haven't been able to complete the testing and F37 and before won't likely switch either.

We would need bootupd to be able to work in those cases as well to be able to add bootupd to them and allow users to update their bootloaders.

See: coreos/fedora-coreos-tracker#1441
See: fedora-silverblue/issue-tracker#434
See: fedora-silverblue/issue-tracker#120

[travier]

Silverblue and friends are now using unified-core and IoT is planning on switching.

[travier]

Fedora Atomic Desktops and Fedora IoT are now built in rpm-ostree unified core mode.