Fix: cpp-common/bt2c/logging.hpp: don't set `_mSelfComp` when creating `Logger` for...

Fix: cpp-common/bt2c/logging.hpp: don't set `_mSelfComp` when creating `Logger` for message iterator

Only one context in `Logger` must be set.  Similarly, when creating a
`Logger` for a component, we don't set `_mSelfCompCls`.

Change-Id: Ie826986d301cb996b594127c72094b90974b8e87
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12197
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>