usb-acl-helper: Remove extra check for NULL
usb-acl-helper: Remove extra check for NULL

g_cancellable_cancel() and g_clear_object() can handle it

Acked-by: Victor Toso <39b7599ff73bbf5db531748ba5250f21fa7120ff@redhat.com>

SPICE/spice-gtk,SPICE/spice-gtk,flexVDI/spice-gtk,flexVDI/spice-gtk,SPICE/spice-gtk,fgouget/spice-gtk,SPICE/spice-gtk,fgouget/spice-gtk,fgouget/spice-gtk