PLANET-6811 Remove default duotones

Ref: https://jira.greenpeace.org/browse/PLANET-6811
Ref: WordPress/gutenberg#40383

---

Our current `theme.json` option doesn't work in 6.0.
You will probably see a validation error in your editor, because `null`
is not yet reflected in the official schema, but it does seem to work
without issues.

theme.json

@@ -8,7 +8,7 @@
       "customGradient": false,
       "defaultGradients": false,
       "defaultPalette": false,
-      "duotone": [],
+      "duotone": null,
       "palette": [
         {
           "name": "Grey 80%",