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/angular2/test/testing/testing_public_spec.ts

@@ -14,7 +14,8 @@ import {
   TestComponentBuilder
 } from 'angular2/testing';
 
-import {Injectable, NgIf, bind} from 'angular2/core';
+import {Injectable, bind} from 'angular2/core';
+import {NgIf} from 'angular2/common';
 import {Directive, Component, View, ViewMetadata} from 'angular2/angular2';
 import {XHR} from 'angular2/src/compiler/xhr';
 import {XHRImpl} from 'angular2/src/platform/browser/xhr_impl';