DEV: Remove unused disabled_plugins checks (#13144)

We now bundle Javascript for each theme/plugin separately, and only ship bundles for enabled plugins to the client. Therefore, these disabled_plugins checks are now redundant, and can be removed.

plugins/styleguide/assets/javascripts/discourse/styleguide-route-map.js.es6

@@ -1,10 +1,4 @@
 export default function () {
-  const { disabled_plugins = [] } = this.site;
-
-  if (disabled_plugins.indexOf("styleguide") !== -1) {
-    return;
-  }
-
   this.route("styleguide", function () {
     this.route("show", { path: ":category/:section" });
   });