home: comma: remove 'COMMA_PKGS_FLAKE' definition

Now that my configurations also set `nixpkgs` in `NIX_PATH`, there's
isn't a need for this to be defined anymore.

home/comma/default.nix

@@ -5,25 +5,11 @@ in
 {
   options.my.home.comma = with lib; {
     enable = my.mkDisableOption "comma configuration";
-
-    pkgsFlake = mkOption {
-      type = types.str;
-      default = "pkgs";
-      example = "nixpkgs";
-      description = ''
-        Which flake from the registry should be used with
-        <command>nix shell</command>.
-      '';
-    };
   };
 
   config = lib.mkIf cfg.enable {
     home.packages = with pkgs; [
       ambroisie.comma
     ];
-
-    home.sessionVariables = {
-      COMMA_PKGS_FLAKE = cfg.pkgsFlake;
-    };
   };
 }