plugins: remove deprecated route registrations

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

plugins/welcome/src/plugin.ts

@@ -19,7 +19,6 @@ import {
   createRoutableExtension,
   createRouteRef,
 } from '@backstage/core';
-import WelcomePageComponent from './components/WelcomePage';
 
 export const rootRouteRef = createRouteRef({
   title: 'Welcome',
@@ -27,8 +26,7 @@ export const rootRouteRef = createRouteRef({
 
 export const welcomePlugin = createPlugin({
   id: 'welcome',
-  register({ router, featureFlags }) {
-    router.addRoute(rootRouteRef, WelcomePageComponent);
+  register({ featureFlags }) {
     featureFlags.register('enable-welcome-box');
   },
 });