fdo#75519: fix chapter cross references

It would be possible to modify the condition in
SwChapterField::ChangeExpansion() to check for rTxtNd.GetNumRule() as
well and make the bugdoc work again, but do we really want to use
"chapter reference" to refer to plain numbering?  Perhaps it would be
better to add a "numbering reference" field type; without investigating
what Word actually does here and what the actual problem is that was
supposedly fixed here (which is not possible given there's no reproducer
document) it's best for now to just revert it.

(regression from commit 6319803762d051c4aa645692cde7245a1bbcf7ee)

Change-Id: I7eb9ea46e42729c53c350ff450d49a1eb84ee8eb
(cherry picked from commit cc0a7699bb79efad488094e33464d4793758ee3f)
Reviewed-on: https://gerrit.libreoffice.org/9271
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>