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/order_management/index.ts

@@ -1,18 +1,16 @@
 import {bootstrap} from 'angular2/bootstrap';
 import {
-  NgIf,
-  NgFor,
   Component,
   Directive,
   Host,
   forwardRef,
   Provider,
   EventEmitter,
-  FORM_DIRECTIVES,
   Injectable,
   Input,
   Output
 } from 'angular2/core';
+import {NgIf, NgFor, FORM_DIRECTIVES} from 'angular2/common';
 
 import {ListWrapper} from 'angular2/src/facade/collection';