ir: do not automatically generate a UUID in bt_ctf_clock_class_create()

ir: do not automatically generate a UUID in bt_ctf_clock_class_create()

A clock class should be created empty, without a generated UUID which is
different each time. To maintain backward compatibility, a CTF writer
clock automatically generates a UUID on creation to set its private
clock class's UUID.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>