Input: acecad - simplify usb_acecad_disconnect()

There is no need to check whether acecad structure is attached to the
interface; if it isn't and we are called we have much bigger problems.
Also no need to call usb_kill_urb() in usb_acecad_disconnect() since
it is being called in usb_acecad_close().

Signed-off-by: Dmitry Torokhov <dtor@mail.ru>