thermal: core: Drop tz field from struct thermal_instance

thermal: core: Drop tz field from struct thermal_instance

After recent changes, it is only used for printing a debug message
in __thermal_cdev_update() which arguably is not worth preserving.

Drop it along with the dev_dbg() statement using it.

Link: https://lore.kernel.org/linux-pm/4a8d8f5a-122d-4c26-b8d6-76a65e42216b@linaro.org
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://patch.msgid.link/4934182.GXAFRqVoOG@rjwysocki.net