Service Worker: Remove worker-no-op.js and use empty-worker.js

Cleanup-only patch. The two files served the same purpose and usage was
split 50/50. I chose empty-worker.js since it fits our *-worker.js
naming pattern.

Review URL: https://codereview.chromium.org/493873003

git-svn-id: svn://svn.chromium.org/blink/trunk@180715 bbb929c8-8fbe-4397-9dbb-9b2b20218538