Clean up the data for navigator.getBattery()

[foolip]

Summary of changes:

• Remove navigator.battery, which has been removed since 2013/2016 in
  Chrome and Firefox respectively.
• Treat BatteryManager as only supported since navigator.getBattery()
  was supported, making the data consistent but technically wrong.
• Simplify Firefox support to only represent when it was on by default
  for web content. Flags before that and chrome-only after is removed.
• Remove claims of prefixes on BatteryManager, the only prefixes here
  have been navigator.mozBattery and navigator.webkitBattery.
• Don't claim it's deprecated, https://w3c.github.io/battery/ is still
  a "real" spec and Chromium hasn't deprecated it.
• Scope the chargingTime/dischargingTime notes to just those entries,
  and treat it as fixed in Chromium 42 (see below).

The chargingTime/dischargingTime issue was fixed in Chromium 42:
https://bugs.chromium.org/p/chromium/issues/detail?id=401553
https://storage.googleapis.com/chromium-find-releases-static/13e.html#13e397e55501bee3903bfe46edfd56f2442c1f11

Data originally from #1102.
There was no evidence for or discussion on the webview_android version
being 40. All existing versions were treated as untrustworthy and data
was mirrored to all Chromium browsers.

[foolip]

Reviewers, there are two things here I expect some might disagree with and are worth flagging.

First, should navigator.battery and its prefixed forms instead be treated as aliases of navigator.getBattery() and merged into that entry? I didn't do this because it was changed from an attributes to an async method, and I'm not sure how to avoid the impression that there was a navigator.battery(). If we ever add types to BCD, this entry would be problematic.

Second, I nuked a lot of notes for Firefox. They don't seem relevant to anyone today, but I'm not sure, should some of it be preserved to help people understand why this was remove from Firefox?

[ddbeck]

First, should navigator.battery and its prefixed forms instead be treated as aliases of navigator.getBattery() and merged into that entry?

No, I think you've made the right choice here. I think it's fine to treat this as a materially different feature which was removed. It so happens that it was removed sufficiently long ago that it's now irrelevant. Let's put it in the 🗑️.

That said, would you mind opening a PR on mdn/content to redirect https://developer.mozilla.org/en-US/docs/Web/API/Navigator/battery to https://developer.mozilla.org/en-US/docs/Web/API/Navigator/getBattery?

Second, I nuked a lot of notes for Firefox. They don't seem relevant to anyone today, but I'm not sure, should some of it be preserved to help people understand why this was remove from Firefox?

I think you made the right call here too. The prefixes are ancient history and would be removed if they were standalone features. The prefs would also be irrelevant by now. The chrome code stuff is off-topic and could also be removed legitimately. What's left makes sense. 👍

I do have one suggestion in a line comment, about the experimental status for all of this.