Revert "Silverblue/Kinoite/Sericea/Onyx: Use unified core mode"
    
    Composes are failing on:
    
    ```
    error: Unable to load SELinux policy from /
    ```
    
    Which comes from https://github.com/coreos/rpm-ostree/blob/main/src/libpriv/rpmostree-core.cxx#L795
    
    Revert this change until we either add selinux-policy to the buildroot
    or fix this in rpm-ostree.
    
    This reverts commit 6d82501bcae74b2db5b785ad56f27aebac5c07f3.
    
    Signed-off-by: Timothée Ravier <tim@siosm.fr>