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/compiler/compiler_benchmark.ts

@@ -14,7 +14,6 @@ import {
   bind,
   provide,
   Provider,
-  NgIf,
   ViewMetadata
 } from 'angular2/core';