Remove ARIA reflection spec in this repo and point to ARIA editor's draft

[mrego]

Attribute reflection is now in the ARIA spec: w3c/aria#1755
And also the HTML spec defines now how this work for Element and FrozenArray: whatwg/html#7934

We could remove https://wicg.github.io/aom/spec/aria-reflection.html and make https://wicg.github.io/aom/spec/#aria-reflection point to https://w3c.github.io/aria/#idl-reflection-attribute-values or https://w3c.github.io/aria/#idl-interface

We could also update the explainer pointing to the proper specs: https://github.com/WICG/aom/blob/gh-pages/aria-reflection-explainer.md