staging: comedi: comedi_test: remove unnecessary del_timer() call

In the "comedi_test" module, the timer function
`waveform_ai_interrupt()` doesn't need to remove the timer from the
timer queue as the caller has already removed it from the queue.  Remove
the call to `del_timer()` in this function.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>