Follow-up cleanup: remove unused method WebServiceWorkerContextClient::workerContextDestroyed()

Follow-up cleanup for
https://codereview.chromium.org/238273004/

BUG=361790
R=tkent@chromium.org

Review URL: https://codereview.chromium.org/246273006

git-svn-id: svn://svn.chromium.org/blink/trunk@172352 bbb929c8-8fbe-4397-9dbb-9b2b20218538