Remove chai and assert from global window

It has caused accidents where 'assert' is used in prod code without an
import, and typescript does not notice the issue. Examples:

https://gerrit-review.googlesource.com/c/gerrit/+/343916
https://gerrit-review.googlesource.com/c/gerrit/+/343475

Removal is in common-test-setup.ts, package.json, and BUILD. All other
ts/js files are just adding the import from @open-wc/testing.

Release-Notes: skip
Change-Id: Ie38c0a52d6a0f8f7f1468e2e8fc9b721d4aef734

polygerrit-ui/app/elements/core/gr-router/gr-router_test.ts

diff --git a/polygerrit-ui/app/elements/core/gr-router/gr-router_test.ts b/polygerrit-ui/app/elements/core/gr-router/gr-router_test.ts
index 02d312f..c8581df 100644
--- a/polygerrit-ui/app/elements/core/gr-router/gr-router_test.ts
+++ b/polygerrit-ui/app/elements/core/gr-router/gr-router_test.ts
@@ -42,6 +42,7 @@
   GenerateUrlChangeViewParameters,
   RepoDetailView,
 } from '../../../utils/router-util';
+import {assert} from '@open-wc/testing';
 
 suite('gr-router tests', () => {
   let router: GrRouter;