Clean up obsolete shadow DOM-related code

* Remove isUnknownPseudoElement() (no user)
* Remove Element.customPseudoId(), use Element.pseudoId()
  Only PseudoElement::customPseudoId() makes sense,
  otherwise NOPSEUDO is returned.

BUG=none
TEST=no layout test failures

Review URL: https://codereview.chromium.org/404363006

git-svn-id: svn://svn.chromium.org/blink/trunk@178750 bbb929c8-8fbe-4397-9dbb-9b2b20218538