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/tree/tree_benchmark.ts

@@ -7,9 +7,9 @@ import {
   ViewContainerRef,
   bind,
   provide,
-  Provider,
-  NgIf
+  Provider
 } from 'angular2/core';
+import {NgIf} from 'angular2/common';
 
 import {ApplicationRef} from 'angular2/src/core/application_ref';
 import {DOM} from 'angular2/src/core/dom/dom_adapter';