Cleanup service worker samples. Remove old workarounds

I've seen several of the service worker samples that have fallbacks, workarounds
for earlier versions of Chrome, and perhaps outdated practices. This can be confusing when someone is trying to learn SW and these extra lines are no longer necessary.

Shall we clean up the examples?

Example that talks about Chrome 49: https://googlechrome.github.io/samples/service-worker/custom-offline-page/

cc @jeffposnick