cpp-common/bt2c: remove lib-str.hpp

cpp-common/bt2c: remove lib-str.hpp

This is unused, and I think it will never be.  If we need something like
this, it will likely be in the form of a `format_as` function used by
the `{fmt}` library.

Change-Id: I81e21d697f6b246c986c5c9126b1177a0d5bcb03
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11759
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>