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/examples/forms/ts/nestedFormArray/nested_form_array_example.ts

@@ -8,7 +8,7 @@
 
 // #docregion Component
 import {Component} from '@angular/core';
-import {AbstractControl, FormArray, FormControl, FormGroup} from '@angular/forms';
+import {FormArray, FormControl, FormGroup} from '@angular/forms';
 
 @Component({
   selector: 'example-app',