staging: comedi: me_daq: remove incomplete analog input command support

The analog input subdevice functions me_ai_do_cmd_test() and me_ai_do_cmd()
are only stubbed in. They both just return 0.

The me_ai_cancel() function does have code to stop any running conversions
but the me_ai_insn_read() function does that also.

Just remove the incomplete functions and remove SDF_CMD_READ from the
subdevice_flags.

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>