user.js: Beacon API - don't bother?

I think we should get rid of 2602, the analytics it can deliver can be obtained and shared elsewhere so there’s no net gain; on the flip side, it might impact performance and we had a couple reports of breakage in the past.

bonus: one less flip, one less API disabled, arguable fingerprint gain.

sources:

• https://developer.mozilla.org/en-US/docs/Web/API/Navigator/sendBeacon
• https://www.w3.org/TR/beacon/#privacy-and-security