Snapshot warning if there are no events in one of the domains

Remove the "no data" check from the UST snapshot code since

1) It would report that no data was recorded in the snapshot even
   if the kernel tracer had recorded events,
2) The kernel consumer does not seem to report such a condition,
   making it hard to perform the check without modifying the
   consumer.

The consumer interfaces could be modified to reliably provide
this information in the future.

However, this warning is not provided in the other tracing
modes, making its usefulness questionable. An empty snapshot is
still a valid trace and users should not always expect a trace
to contain events. Moreover, "warning" that no data was produced
implies an error may have occured and is probably as confusing
as opening an empty trace to the average user.

Fixes #1012

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>