cpp-common: remove `cfg-*.hpp` and `log-cfg.hpp`

cpp-common: remove `cfg-*.hpp` and `log-cfg.hpp`

Those are not used currently and I plan to replace them with something
more woke for C++.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ied48a1f6a5c32426a8d85e32c6a5682c48dd4e42
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11389
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
CI-Build: Simon Marchi <simon.marchi@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>