staging: comedi: 8253.h: remove the unused i8253_cascade_ns_to_timer_*()

None of the comedi drivers use the i8253_cascade_ns_to_timer_2div_old()
or i8253_cascade_ns_to_timer_power() helpers to calculate the cascaded
divisors. Remove them to avoid any confusion.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>