Clean up SdrModel::MigrateItemSet

* No need to support null pNewModel argument (the one call site that did pass in
  null explicitly in sd/source/ui/func/fupate.cxx can just as well pass in
  mpDoc, and all existing call sites are guaranteed to pass in a non-null
  value).

* With that changed, SdrModel::MigrateItemSet can be static.  (At least in
  JunitTest_forms_unoapi it could happen that the call from
  svx/source/sdr/properties/attributeproperites.cxx called MigrateItemSet on a
  null GetSdrObject.GetModel(), which was undefined behavior even though it was
  harmless.)

Change-Id: Idface48da7e889c6e5768d0e49bc67c88b4c3ec4