Remove duplicate AudioScheduledSourceNode start/stop/onended entries

[foolip]

AudioBufferSourceNode is as old as the Web Audio API itself, and
originally had these members. OscillatorNode and ConstantSourceNode were
added later and also had these members. Later, AudioScheduledSourceNode
was introduced and the members were moved up to that prototype. Only an
overload start() method remained on AudioBufferSourceNode.

Summary of changes:

• Remove the members no longer in spec/implementations.
• Mark the members of AudioScheduledSourceNode as supported since the
  earliest time they were supported on AudioBufferSourceNode, which is
  the earliest of the concrete interfaces.
• Mark AudioScheduledSourceNode as partially supported from the
  versions AudioBufferSourceNode were introduced until the
  AudioScheduledSourceNode interface was added.

Introduction of AudioScheduledSourceNode in each browser/engine:

Chromium 57:
https://bugs.chromium.org/p/chromium/issues/detail?id=661207
https://storage.googleapis.com/chromium-find-releases-static/009.html#00971f38908388728f49cd5127b9c6c6761d035f

Firefox 53:
https://bugzilla.mozilla.org/show_bug.cgi?id=1324568
mozilla/gecko-dev@bd93c7b

WebKit trunk 610.1.20, which was Safari 14:
https://bugs.webkit.org/show_bug.cgi?id=214381
https://trac.webkit.org/changeset/264538/webkit

There was an AudioSourceNode parent interface in WebKit, but it never
had any members.

These tests were used to find or confirm some version numbers:
https://staging-dot-mdn-bcd-collector.appspot.com/tests/api/AudioScheduledSourceNode

(It uses a AudioBufferSourceNode, as the oldest concrete type now
inheriting from AudioScheduledSourceNode.)

For start/stop, Chrome 23 and 24 were tested to confirm.

WebKit commit for that at trunk version 537.12:
https://trac.webkit.org/changeset/129260/webkit
https://trac.webkit.org/browser/webkit/trunk/Source/WebCore/Configurations/Version.xcconfig?rev=129260

For onended, Chrome 29 and 30 were tested to confirm.

WebKit commit for that at trunk version 537.44:
https://trac.webkit.org/changeset/150905/webkit
https://trac.webkit.org/browser/webkit/trunk/Source/WebCore/Configurations/Version.xcconfig?rev=150905

That also introduced support for the event itself.

Edge 13 was tested to confirm support for onended/start/stop, and it was
assumed to also be in Edge 12 since most of the Web Audio API was.

Firefox 24 and 25 tested were tested to confirm Firefox data.

Fixes #6656.

[foolip]

I think this is my longest commit message in BCD yet :)

[Elchi3]

Thanks, this follows the pattern of using partial_implementation nicely for prototype moves. It would be nice if you could write a data guideline for this now that we've successfully used this in at least two scenarios.

I've reviewed this by comparing with the current IDL files and it makes sense structurally. For the all versions numbers I just glanced over them and I think they make sense.