core: remove service-worker audit

[adamraine]

Justification from internal email discussion:

• A service worker is no longer required for PWA insatiability in Chrome
• Even before the SW requirement was removed, this audit was mostly redundant with the installable manifest audit
• Many users see this audit failing and add a pointless service worker just to get it to pass, even though this can hurt performance.

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Edit: I may have jumped the gun on removing this audit. We should land #15253 first. Done

Keeping the ServiceWorker gatherer around like we do with IFrameElements. I'm open to removing completely though.

Some SW related drive by updates in core/audits/installable-manifest.js

#14909

[adamraine]

This is the only audit we check in this smoke test, so it doesn't seem useful anymore.

[khmyznikov]

@adamraine how to re-enable this audit manually?

[adamraine]

It's not possible ATM, but you could create a custom audit with the same implementation.