[web-audio-api] Remove empty AudioSourceNode interface (#195)

> Originally reported on W3C Bugzilla [ISSUE-21511](https://www.w3.org/Bugs/Public/show_bug.cgi?id=21511) Tue, 02 Apr 2013 12:18:48 GMT
> Reported by Olivier Thereaux
> Assigned to 

Per discussion at Audio WG f2f 2013-03-26.

The AudioSourceNode interface (Section 4.3 in current draft) is not serving any purpose other than providing an abstract class, which is too prescriptive for implementations. The group agreed to remove it.

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https://github.com/WebAudio/web-audio-api/issues/195