[whatwg] Elements should be removed from the past names map once it's no longer associated with the form element

[Ryosuke Niwa]

Hi all,

I propose to change the specification to remove any elements that are no longer associated with the form from the past names map:
http://www.whatwg.org/specs/web-apps/current-work/multipage/forms.html#past-names-map

It's strange for elements to linger around forever in the past names map after the element get associated with a new form element.  Since IE didn't support form attribute for a long time (or maybe still hasn't?), I don't think there is any compatibility concern.

The change makes the elements associated with the form element more consistent and easier to reason about.  We can make form[x].form === form an invariant for every x such that form[x] is a form control element.

- R. Niwa