aria-selected is not generally necessary on single selection containers

I was looking at the ARIA 1.1 combobox example, and here is some feedback a
markup simplification that aligns with the specs:
Using aria-selected at all is generally redundant in a list of options, or
any single selection container. Using either focus or aria-activedescendant
is enough.

See here in the ARIA spec:
https://www.w3.org/TR/wai-aria-1.1/#aria-selected
"Single-selection containers where the currently focused item is not
selected. The selection normally follows the focus, and is managed by the
user agent."
And in CORE-AAM: "In the single selection case, selection follows focus".

There are rare cases in single selection containers where the author wants
nothing to be selected, even though something is focused. In this case the
author should use aria-selected="false" on the focused item.

Firefox has always gotten this right, but Chrome had it wrong. I just fixed
this for tomorrow's Canary build. See
https://bugs.chromium.org/p/chromium/issues/detail?id=846089 ("NVDA reading
"not selected" in combobox list boxes").

Bottom line, I believe the lines setting aria-selected should be removed
from the combo box examples. I haven't checked other places in the ARIA
practices. And I haven't checked what happens with every browser + screen
reader combination. (It's generally up to the browser to do the right
thing.)

Aaron