Input: kbtab - simplify kbtab_disconnect()

There is no need to check whether kbtab 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 kbtab_disconnect() since it
is being called in kbtab_close().

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