cpp-common/bt2s: use a `using` declaration

cpp-common/bt2s: use a `using` declaration

This is exactly the purpose of a `using` declaration, to make the symbol
from the `bpstd`/`nonstd` namespace accessible for unqualified lookup as
if declared in the `bt2s` namespace.

The code is much simpler.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I8348f409a463b85f5b2ed1f1fd8f61adb404a6d9
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11727
Tested-by: jenkins <jenkins@lttng.org>
CI-Build: Simon Marchi <simon.marchi@efficios.com>