staging: comedi: ni_mio_common: remove INSN_CONFIG_ANALOG_TRIG support code

None of the boardinfo that uses this code sets the 'has_analog_trig' flag so
ni_ai_config_analog_trig() always returns -EINVAL.

Also, the private data 'atrig_low', 'atrig_high', and 'atrig_mode' values
that are set by ni_ai_config_analog_trig() are never used.

Remove the incomplete INSN_CONFIG_ANALOG_TRIG support along with the unused
boardinfo and private data members.

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>