platform: Remove stale options from config.mk files

This patch removes stale options from config.mk files of
Ariane FPGA and QEMU virt platform support.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>