Use an AbortController instead of signaling abort on AbortSignal

[shaseley]

For https://github.com/whatwg/dom/issues/1194 (and follow-up from https://github.com/whatwg/streams/pull/1277).

• Replace WritableStreamDefaultController's [[signal]] with [[abortController]], using this abort controller's signal where [[signal]] was used. Signal abort on [[abortController]] rather than the [[signal]].
• Remove the note/warning about specs not signaling abort on the WritableStream's signal, since this won't be allowed once AbortSignal's "signal abort" no longer exported.

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[shaseley]

@annevk how does this look?

[MattiasBuelens]

Latest build failure looks like a problem with Bikeshed itself. I've opened speced/bikeshed#2561.

[MattiasBuelens]

The new Bikeshed version fixes the build issue, so I think we're good to go! 🙂