staging: comedi: addi_apci_3120: remove apci3120_ai_insn_config()

This function does not follow the comedi API for (*insn_config) functions.
It's also no long needed. Just remove it.

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>