staging: comedi: addi_apci_3501: remove i_APCI3501_ConfigDigitalOutput()

This function is the digital output subdevice 'insn_config' operation.
It does not follow the comedi API and the digital output channels on
this board are not configurable. Just remove this broken 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>