Don't automatically generate a clock UUID in _bt_ctf_clock_create

While the public clock constructor still generates a UUID,
it should not be done when creating a clock during TSDL metadata
parsing.

This makes it possible to use bt_ctf_clock_get_uuid to check
if a UUID has already been set during clock definition parsing.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>