Revert "Implement cloneNode for Document"

This reverts r155787

There is more state that needs to be copied/initialized.

BUG=258146
TBR=abarth

Review URL: https://chromiumcodereview.appspot.com/23103005

git-svn-id: svn://svn.chromium.org/blink/trunk@156125 bbb929c8-8fbe-4397-9dbb-9b2b20218538