lib: Reset libbabeltrace2 to SONANE 0

Following the rename of the library, reset the SONAME to zero and
decouple this from the project versioning which is not tied directly to
the library ABI.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Change-Id: If3a3c890674d6236cfe33b5f3f1e3fe4229c5691
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1404
Tested-by: jenkins
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>