Removed EventEmitter inheritance in frontend routers

refs https://linear.app/tryghost/issue/CORE-104/decouple-frontend-routing-events-from-urlserver-events

- Now that routers don't emit any events there's no need to extend EventEmitter in the router's base class
- Less inheritance the lighter the runtime and the cleaner the code!

core/frontend/services/routing/ParentRouter.js

@@ -9,7 +9,6 @@
 
 const debug = require('@tryghost/debug')('routing:parent-router');
 
-const EventEmitter = require('events').EventEmitter;
 const express = require('../../../shared/express');
 const _ = require('lodash');
 const url = require('url');
@@ -46,10 +45,8 @@ function GhostRouter(options) {
     return innerRouter;
 }
 
-class ParentRouter extends EventEmitter {
+class ParentRouter {
     constructor(name) {
-        super();
-
         this.identifier = security.identifier.uid(10);
 
         this.name = name;