Remove internalSubset from DocumentType

Remove internalSubset from DocumentType, since even though it is being read in MarkupAccumulator it is not being set anywhere, and also it is scheduled to be removed from latest DOM spec.

Review URL: https://codereview.chromium.org/282823004

git-svn-id: svn://svn.chromium.org/blink/trunk@174043 bbb929c8-8fbe-4397-9dbb-9b2b20218538