hw/i386/pc: Remove PC_MACHINE_DEVMEM_REGION_SIZE

hw/i386/pc: Remove PC_MACHINE_DEVMEM_REGION_SIZE

There are no remaining users in the tree. Libvirt never used that
property and a quick internet search revealed no other users.

Further, we renamed that property already in commit f2ffbe2b7dd0
("pc: rename "hotplug memory" terminology to "device memory"") without
anybody complaining.

So let's just get rid of it.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Richard Henderson <richard.henderson@linaro.org>
Cc: Eduardo Habkost <eduardo@habkost.net>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20230623124553.400585-9-david@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>