Remove loadExtensionURL return value

Never used anywhere and will make a follow-up commit more complicated

src/extension-support/extension-manager.js

@@ -339,7 +339,7 @@ class ExtensionManager {
         if (e) {
             workerInfo.reject(e);
         } else {
-            workerInfo.resolve(id);
+            workerInfo.resolve();
         }
     }