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/router/test/url_serializer.spec.ts

@@ -7,7 +7,7 @@
  */
 
 import {PRIMARY_OUTLET} from '../src/shared';
-import {DefaultUrlSerializer, UrlSegmentGroup, decode, encode, serializePath} from '../src/url_tree';
+import {DefaultUrlSerializer, UrlSegmentGroup, encode, serializePath} from '../src/url_tree';
 
 describe('url serializer', () => {
   const url = new DefaultUrlSerializer();