dmx: Remove some not-very-interesting debug prints

gcc/glibc think the snprintf in dmxExecOS() might truncate. Yes, it
might, and we also don't care. Just delete all this.

Signed-off-by: Adam Jackson <ajax@redhat.com>
Acked-by: Keith Packard <keithp@keithp.com>
(cherry picked from commit d6db6681)