Code cleanup: Removed dead code + use DU.isNewElt everywhere

* Removed some dead code from WikitextSerializer.serializedAttrVal
  and DU.getAttributeShadowInfo -- that gets rid of inconsistency
  from those methods where we returned a string sometimes and an
  object other times. That's just asking for trouble.

* Removed a few lurker node.hasAttribute('data-parsoid') and friends
  to use DU.isNewElt(node) instead.

Change-Id: I9bc00eb8c2951c23d495c6d7a99b04ecc6620db4
Merge "Code cleanup: Removed dead code + use DU.isNewElt everywhere"