staging: comedi: addi_apci_3120: remove private data 'ui_AiReadData'

This member of the private data was used to return analog input samples that
were acquired for the (*insn_read) using interrupts. The interrupt support
code for the (*insn_read) has been removed. Remove this unused member from
the private data.

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>