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/model_driven_forms/index.ts

@@ -7,12 +7,9 @@ import {
   NgFormModel,
   FormBuilder,
   NgIf,
-  NgFor,
-  Component,
-  Directive,
-  View,
-  Host
-} from 'angular2/core';
+  NgFor
+} from 'angular2/common';
+import {Component, Directive, View, Host} from 'angular2/core';
 
 import {RegExpWrapper, print, isPresent} from 'angular2/src/facade/lang';