Clean up DocumentType now that it always has a Document

DocumentType always has an ownerDocument now so we can clean up its
insertedInto and removedFrom callbacks.

When I was cleaning this up I also realized that DocumentType::entities() and
notations() always return null and were removed in DOM4 so we can remove their
storage from DocumentType.

Review URL: https://chromiumcodereview.appspot.com/23534003

git-svn-id: svn://svn.chromium.org/blink/trunk@156818 bbb929c8-8fbe-4397-9dbb-9b2b20218538