Atomic/partial updates to nested docs should not assume _route_ param is the root ID

In SOLR-14923, doing an atomic/partial update to a child doc requires indicating what the root doc ID is.  Formerly Solr tried to figure this out automatically, but that had costly implications.   SOLR-14923 adds a temporary hack that assumes that the _route_ paramter is the root ID, if the _root_ field is not on the doc.  But that's not necessarily accurate, so it should be removed.