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/benchmarks/src/naive_infinite_scroll/app.ts

@@ -2,7 +2,7 @@ import {isPresent} from 'angular2/src/facade/lang';
 import {getIntParameter, bindAction} from 'angular2/src/testing/benchmark_util';
 import {TimerWrapper} from 'angular2/src/facade/async';
 import {ScrollAreaComponent} from './scroll_area';
-import {NgIf, NgFor} from 'angular2/core';
+import {NgIf, NgFor} from 'angular2/common';
 import {DOM} from 'angular2/src/core/dom/dom_adapter';
 import {document} from 'angular2/src/facade/browser';