staging: comedi: drivers: addi_apci_3501: Removed variables that is never used

Variable was assigned a value that was never used.
I have also removed all the code that thereby serves no purpose.

This was found using a static code analysis program called cppcheck

Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>