TextTemplatePlugin references non-existing diff.css

When editing a document with a Text field, this can be seen (once):

WARN  [org.apache.wicket.request.resource.PackageResource.sendResourceError():390] resource [path = org/hippoecm/frontend/plugins/standards/diff/diff.css, style = null, variation = null, locale = null]: Unable to find resource (status=404)

TextTemplatePlugin has a CssResourceReference(HtmlDiffModel.class, "diff.css") but the diff.css was removed in commit "CMS-10656 Move diff CSS to SCSS" (in Hippo 12 and backported to Hippo 11).