Remove didNotifySubtreeInsertionsToDocument.

This hook existed because some elements wanted to run script inside
::insertedInto, but that wasn't a safe time. Now nothing can run script
inside there so we don't need this extra hook.

HTMLStyleElement's usage of this hook is not clear, but it seems like
it might have related to a pre-lazy attach bug.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/758623002