Strip builder-corrected placeholder metas in editMode

* mw:Placeholders added to track stripped tags are present to RT
  those tags back in roundtrip testing mode.  However, in edit more
  they are currently ignored and introduce RT errors as a result.

* I changed mw:Placeholder type for those to mw:Placeholder/StrippedTag

* For now, I added code to strip them from the DOM before sending
  it out to VE.  But, an alternative fix would be to leave them in
  and fix the serializer to ignore those mw:Placeholders in edit mode.

  In future, if the VE has a use for this information or we are able
  to safely exploit them in the serializer, we can change this, but
  for now, better to get rid of that noise.

* Fixes the following test case

------------------
$ echo "'''''x'''''" | node parse --wt2wt --editMode true
'''''x'''''
------------------

Change-Id: Ib6f1a8c824725158ecaf1a396d8da32ee627f9b3