cleanup(core): stop reexporting angular2/common from angular2/core

All common directives, forms, and pipes have been moved out of angular2/core,
but we kept reexporting them to make transition easier.

This commit removes the reexports.

BREAKING CHANGE

Before

import {NgIf} from 'angular2/core';

After

import {NgIf} from 'angular2/common';

Closes #5362

modules/playground/src/async/index.ts

@@ -1,5 +1,6 @@
 import {bootstrap} from 'angular2/bootstrap';
-import {NgIf, Component, View} from 'angular2/core';
+import {Component, View} from 'angular2/core';
+import {NgIf} from 'angular2/common';
 import {TimerWrapper} from 'angular2/src/facade/async';
 
 @Component({selector: 'async-app'})