meta: Don't use NixOS networks

Networks are a way to start multiple NixOS virtual machines at the same
time. Although cute, networks seem to be used more for "test scripts"
and ultimately don't support Nix flakes. This results in package
versions being outdated and the inability to use our existing
home-manager input and other flakes.

Multiple nixosConfigurations, on the other hand, enable us to setup
entire computers that don't *have* to be virtual machines, but can be
virtual machines if we want them to be. Additionally, it becomes trivial
to only run the configurations you want to run, without having to worry
about everything being tied to everything else. Finally, persistence is
optional and the resulting .qcow2 file is quite small.

Ultimately, Nix flakes are a more flexible solution to the older
nixos-build-vms command, and should be preferred over it in pretty much
all cases. To reiterate, if you're using flakes, there's no reason to
use the outdated nixos-build-vms command, which won't have the same
package versions as the ones in your flake.

networks/default.nix

@@ -1,26 +0,0 @@
-{
-  plasma = { pkgs, ... }: {
-    imports = [
-      ../modules/virtualization.nix
-      ../modules/user.nix
-      ../modules/plasma.nix
-      ../modules/pipewire.nix
-    ];
-    environment.systemPackages = with pkgs; [
-      kitty
-      librewolf
-    ];
-  };
-  gnome = { pkgs, ... }: {
-    imports = [
-      ../modules/virtualization.nix
-      ../modules/user.nix
-      ../modules/gnome.nix
-      ../modules/pipewire.nix
-    ];
-    environment.systemPackages = with pkgs; [
-      kitty
-      librewolf
-    ];
-  };
-}