configs/qemu-ppc-virtex-ml507: remove stray qemu-system entries

Removed unused config entries that sneaked in from commit
a8956b2b

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>