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/common/test/pipes/slice_pipe_spec.ts

@@ -9,7 +9,6 @@
 import {CommonModule, SlicePipe} from '@angular/common';
 import {Component} from '@angular/core';
 import {TestBed, async} from '@angular/core/testing';
-import {browserDetection} from '@angular/platform-browser/testing/browser_util';
 import {expect} from '@angular/platform-browser/testing/matchers';
 
 export function main() {