RTF writing now uses SfxStyleSheetIterator to iterate over style sheets

Change-Id: I9d5e86eecf1849d8ae7f9e7052c434844e27ddfd
Reviewed-on: https://gerrit.libreoffice.org/8481
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>