Fix: trace-ir.hpp: spurious reference on object creation

We create a variant field class, which initially has refcount == 1.  We
then call `.shared()`, which creates another reference, bringing the
refcount to 2.

Fix that by using the `Shared::createWithoutRef()` method.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: Idfe8184497a00d3192e390017d7717c7ff9a3344
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8187
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10830
Tested-by: jenkins <jenkins@lttng.org>
CI-Build: Philippe Proulx <eeppeliteloop@gmail.com>