Remove visual noise from starmath

Change-Id: I45a716abba68521c5ebc203eed205e717efead7b
Reviewed-on: https://gerrit.libreoffice.org/8315
Tested-by: Caolán McNamara <caolanm@redhat.com>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>