Change handling of
is="" in element creation and cloning
Part of fixing https://github.com/whatwg/html/issues/3402. This changes two things:
* It fixes the cloning steps to consult the internal is value, instead of the
is="" content attribute.
* It removes the step from
createElementNS() that would insert an
is="" attribute in the created element, as we've now decided that
is="" is a way of communicating to the parser, and not a general-purpose indicator of customized built-in-ness.
See also https://github.com/whatwg/html/pull/3444 for the other half of the fix.