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';
 

modules/benchmarks/src/naive_infinite_scroll/cells.ts

@@ -1,6 +1,6 @@
 import {ListWrapper, Map} from 'angular2/src/facade/collection';
 import {Company, Opportunity, Offering, Account, CustomDate, STATUS_LIST} from './common';
-import {NgFor} from 'angular2/core';
+import {NgFor} from 'angular2/common';
 
 import {Component, Directive, View} from 'angular2/angular2';
 

modules/benchmarks/src/naive_infinite_scroll/scroll_area.ts

@@ -14,7 +14,7 @@ import {
 } from './common';
 import {generateOfferings} from './random_data';
 import {ScrollItemComponent} from './scroll_item';
-import {NgFor} from 'angular2/core';
+import {NgFor} from 'angular2/common';
 
 @Component({
   selector: 'scroll-area',