nixos/suites/gaming: use Retroarch module and reduce the cores

In preparation for additional hosts that will be used exclusively as a
home media entertainment system which will have limited capabilities in
the first place.

modules/nixos/_private/suites/gaming.nix

@@ -15,20 +15,17 @@ in
   # Just don't ask where you can sail getting the games. :)
   config = lib.mkIf cfg.enable (lib.mkMerge [
     {
-      environment.systemPackages = with pkgs; [
       # All-around emulator. Also what I'm mainly using for quickly
       # initializing sessions.
-        (retroarch.override {
-          cores = with libretro; [
+      programs.retroarch = {
+        enable = true;
+        cores = with pkgs.libretro; [
           bsnes-hd
           desmume
-            dolphin
           dosbox-pure
-            pcsx2
           ppsspp
         ];
-        })
-      ];
+      };
 
       # Setup the go-to platform for Linux gaming. Most of the
       # games should work now with Proton integration.