error.c: clean-up: remove dead error cause clean-up code

error.c: clean-up: remove dead error cause clean-up code

Coverity warns of logically dead code in error.c:
  1506395 Logically dead code
  The indicated dead code may have performed some action; that action will never occur.

  In bt_error_append_cause_from_component_class: Code can never be reached because of a logical contradiction (CWE-561)

`cause` is indeed always NULL at the `end` label since it either failed
to be allocated or its ownership was transferred to the `causes` array.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ie1c746b9fe6d6202cd6c3baaef39516a5e902471
Reviewed-on: https://review.lttng.org/c/babeltrace/+/9614
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>