usb-bot: Drop not needed "allow_hotplug = 0"

Drop useless hack that disables hotplug on bus by setting
"allow_hotplug = 0". Even if bus is hotpluggable, It won't
be possible to add another SCSI device to bus since its
realization will fail early with error "no free target"
in scsi_qdev_realize() method.

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