chore(lint): remove unused imports (#11923)

This was done automatically by tslint, which can now fix issues it finds.
The fixer is still pending in PR https://github.com/palantir/tslint/pull/1568
Also I have a local bugfix for https://github.com/palantir/tslint/issues/1569
which causes too many imports to be deleted.

modules/@angular/forms/test/validators_spec.ts

@@ -7,7 +7,7 @@
  */
 
 import {fakeAsync, tick} from '@angular/core/testing';
-import {afterEach, beforeEach, ddescribe, describe, expect, iit, it, xit} from '@angular/core/testing/testing_internal';
+import {describe, expect, it} from '@angular/core/testing/testing_internal';
 import {AbstractControl, FormControl, Validators} from '@angular/forms';
 import {Observable} from 'rxjs/Observable';