Remove unnecessary sinon import

`sinon` is exposed as a global in the test environment so it doesn't
need to be imported. Additionally some of the dependencies of Sinon
(samsam, nise) cause warnings about circular dependencies and use of
`eval` when bundled by Rollup.

src/sidebar/util/test/oauth-client-test.js

 import fetchMock from 'fetch-mock';
-import sinon from 'sinon';
 
 import FakeWindow from '../../test/fake-window';
 import OAuthClient, { TokenError, $imports } from '../oauth-client';