docs: removing ServerTransferStateModule as is deprecated

Since the latest universal libraries state that ServerTransferStateModule is being deprecated and apparently does not need to be used, it can be removed from the docs and also is not present in the sample universal code available to be downloaded.

Author: CarlosTorrecillas

docs/transfer-http.md

@@ -26,17 +26,17 @@ import {TransferHttpCacheModule} from '@nguniversal/common';
 export class AppBrowserModule() {}
 ```
 
-Then, import `ServerTransferStateModule` in your Server module.
+You don't have to import anything extra in your AppServerModule.
 
 ```ts
 import { NgModule } from '@angular/core';
-import { ServerModule, ServerTransferStateModule } from '@angular/platform-server';
+import { ServerModule } from '@angular/platform-server';
 
 import { AppModule } from './app.module';
 import { AppComponent } from './app.component';
 
 @NgModule({
-  imports: [AppModule, ServerModule, ServerTransferStateModule],
+  imports: [AppModule, ServerModule],
   bootstrap: [AppComponent],
 })
 export class AppServerModule {}