platform: Remove qemu/virt directory

The OpenSBI generic platform works perfectly fine on the QEMU virt
machine so let's remove dedicated QEMU virt machine platform from
OpenSBI.

All QEMU virt machine related documentation in OpenSBI will now
suggest using OpenSBI generic platform.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>