Use SfxStyleSheetIterator to iterate over style sheets in sd/stlsheet.cxx

It is not possible to access n#708518. We must hope that this
is not a regression

Change-Id: I4500d72cc7d6fc488fb015aa8b7907238fa09ac0
Reviewed-on: https://gerrit.libreoffice.org/8484
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>