remove the nix-specific non-Flatpak code

i'll move this into the source proper

default.nix

@@ -21,17 +21,6 @@ stdenv.mkDerivation (final: with final; {
 
   src = ./.;
 
-  postPatch = ''
-    # see: <https://github.com/mdwalters/lemonade/issues/9>
-    substituteInPlace src/main.py \
-      --replace \
-        "{os.environ['XDG_RUNTIME_DIR']}/app/ml.mdwalters.Lemonade/cache" \
-        "{os.environ['HOME']}/.cache/ml.mdwalters.Lemonade" \
-      --replace \
-        'os.path.join(f"{os.environ['"'"'XDG_RUNTIME_DIR'"'"']}/app/ml.mdwalters.Lemonade", "cache")' \
-        'os.path.join(f"{os.environ['"'"'HOME'"'"']}/.cache", "ml.mdwalters.Lemonade")'
-  '';
-
   nativeBuildInputs = [
     desktop-file-utils
     gobject-introspection