SdrFormatter: remove unused attribute

Change-Id: I70633d3a4cea72c21a644e5a2f25865d8ec52122
Reviewed-on: https://gerrit.libreoffice.org/12227
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>