Can we drop 6.1.1.1 ARIA of the Reading Systems specification?

I'm not sure this adds anything to 3.2.9 Requirements related to ARIA and to platform accessibility APIs of the HTML spec.

The HTML spec references HTML-AAM, which in itself makes it more specific than our section about what exposing ARIA means. Our section is not so much an extension as a less specific repetition.

Looking back at the old HTML 5.0 spec at the time we released EPUB 3, there wasn't a similar support section, only some of what became ARIA in HTML. That's no doubt why it was originally added, but it appears to have outlived its usefulness.

/cc @avneeshsingh @GeorgeKerscher @clapierre