rose-pine-gtk: remove reference to nonexistent variable variant

pkgs/data/themes/rose-pine-gtk/default.nix

···
   gtk_engines,
 }:
 stdenv.mkDerivation rec {
-  pname = "rose-pine-${variant}-gtk-theme";
+  pname = "rose-pine-gtk-theme";
   version = "unstable-2022-09-01";
 
   src = fetchFromGitHub {