This reverts parts of commit d87c4e1a72 from @Mic92
After switching from nixos-23.11 to nixos-unstable, I got the following error:
```
$ nixos-rebuild switch --flake nixos/#digitalocean
...
error:
Failed assertions:
- You must set the option ‘boot.loader.grub.devices’ or 'boot.loader.grub.mirroredBoots' to make the system bootable.
```
In my nixos config I don't set boot.loader.grub.devices or boot.loader.grub.mirroredBoots explicitly.
It seems like the values are not really defaulted via mkDefault? I am still relatively new to Nix, not sure how to fix this properly, so I am proposing to revert this change.
168793f151