Remove `skip-string-normalization` in Python formatter config

Initially, this option was added to follow existing coding style of
the project. It was recently agreed upon that relying on `black`
completely would be simpler.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: Ic4f7fc672502d512c10f107c254cec18c28a7710
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8518
CI-Build: Simon Marchi <simon.marchi@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>