Brompton: remove family-css-variable

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

@@ -360,7 +360,6 @@ $config-header: (
 				"fallback": map-deep-get($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", "sm"),
 			"weight": 900,
 			"line-height": 1,