Take advantage of headerGetInstance() in rpmte

Take advantage of headerGetInstance() in rpmte
- rpmte db_instance and dboffset are exactly the same thing, unify
- pull rpmte->db_instance from header directly, ignore dboffset argument
  to rpmteNew() (removing the arg would just break api needlessly, otoh
  it's unlikely that anything outside rpm actually uses rpmteNew())
- eliminate the now unnecessary added/removed union from te struct
- rpmteDBOffset() is now just an alias for rpmteDBInstance(), deprecate?