Exford: remove family-css-variable

exford/sass/_config-child-theme-deep.scss

@@ -359,7 +359,6 @@ $config-header: (
 				"fallback": map-deep-get($config-global, "font", "family", "primary", "fallback"),
 				"css-var": map-deep-get($config-global, "font", "family", "primary", "css-var"),
 			),
-			"family-css-variable": '--font-headings',
 			"size": map-deep-get($config-global, "font", "size", "base"),
 			"weight": 700,
 			"line-height": 1,