Clened up routing module API from unused exports

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

- CollectionRouter and TaxonomyRouter getters were exposed for no good reason. The less we expose the cleaner it is to understand possible dependencies the mocule creates

core/frontend/services/routing/index.js

@@ -6,18 +6,10 @@ module.exports = {
     routerManager: routerManager,
 
     get registry() {
-        return require('./registry');
+        return registry;
     },
 
     get helpers() {
         return require('./helpers');
-    },
-
-    get CollectionRouter() {
-        return require('./CollectionRouter');
-    },
-
-    get TaxonomyRouter() {
-        return require('./TaxonomyRouter');
     }
 };