Cleanup: flt.lttng-utils.debug-info: Dead assignment

Since the `event_get_payload_build_id_length()` and
`event_get_payload_build_id_value()` functions, can't fail, make them
return void.

Scan-build report:
  Value stored to 'ret' is never read

Reported-by: Scan-Build - Dead assignment
Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I950d0d327c19578d9b715e67417afd05f2ac2645
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2201
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>