digital-ocean: revert "make it easier to use disko"

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.

nixos/modules/virtualisation/digital-ocean-config.nix

@@ -41,7 +41,7 @@ with lib;
         kernelParams = [ "console=ttyS0" "panic=1" "boot.panic_on_fail" ];
         initrd.kernelModules = [ "virtio_scsi" ];
         kernelModules = [ "virtio_pci" "virtio_net" ];
-        loader.grub.devices = lib.mkDefault ["/dev/vda"];
+        loader.grub.devices = ["/dev/vda"];
       };
       services.openssh = {
         enable = mkDefault true;