virtio: drop default for virtio-mem

There's no special reason why virtio-mem needs a default that's
different from what kconfig provides, any more than e.g. virtio blk.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: David Hildenbrand <david@redhat.com>

drivers/virtio/Kconfig

@@ -105,7 +105,6 @@ config VIRTIO_BALLOON
 
 config VIRTIO_MEM
 	tristate "Virtio mem driver"
-	default m
 	depends on X86_64
 	depends on VIRTIO
 	depends on MEMORY_HOTPLUG