staging: comedi: rti800: remove interrupt code

Interrupts are not enabled, or used, in this driver. Remove the
{request,free}_irq() as well as the dummy interrupt function.

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