qdev: Drop legacy hotplug fields/methods

It removes not needed anymore BusState::allow_hotplug field and
DeviceClass::unplug callback.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>