staging: comedi: usbduxsigma: remove unused "convert" timing for AO

The `cmdtest` and `cmd` handlers for the AO subdevice
(`usbduxsigma_ao_cmdtest()` and `usbduxsigma_ao_cmd()`) support "scan"
timing of commands with all channels updated every "scan" period.  There
is some disabled code to use "convert" timing in high speed mode.  That
would allow channels to be updated sequentially every "convert" period.
Since that code is incomplete and currently disabled, remove it to
simplify the existing code.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: Bernd Porr <mail@berndporr.me.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>