Remove dead code of V8NodeCustom.

We should not have Entity/Notation node objects. Their constructors are private
and they don't have factory functions.

TEST=none; no behavior changes.
BUG=

Review URL: https://chromiumcodereview.appspot.com/23447019

git-svn-id: svn://svn.chromium.org/blink/trunk@157640 bbb929c8-8fbe-4397-9dbb-9b2b20218538