Use SfxStyleSheetIterator to iterate over SfxStyles in sd/stlfamily.cxx

Change-Id: I535b47de0fb4e66dc4b02d4c75b6631eb7d87ed4
Reviewed-on: https://gerrit.libreoffice.org/8482
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>