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

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

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