staging: comedi: usbduxsigma: remove AI scan_begin_src == TRIG_FOLLOW

The AI subdevice `cmdtest` handler `usbduxsigma_ai_cmdtest()` ensures
that `cmd->scan_begin_src == TRIG_TIMER` by the end of step 2 of the
command checking code, so assume that this is the case for step 3
onwards and remove the redundant 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>