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/core/debug/debug_element_spec.ts

@@ -18,7 +18,8 @@ import {DOM} from 'angular2/src/core/dom/dom_adapter';
 
 import {PromiseWrapper, EventEmitter, ObservableWrapper} from 'angular2/src/facade/async';
 
-import {Injectable, NgFor, NgIf} from 'angular2/core';
+import {Injectable} from 'angular2/core';
+import {NgFor, NgIf} from 'angular2/common';
 import {Scope} from 'angular2/core';
 import {By} from 'angular2/platform/browser';