tools/lib/thermal: Fix thermal_sampling_exit()

thermal_sampling_init() suscribes to THERMAL_GENL_SAMPLING_GROUP_NAME group
so thermal_sampling_exit() should unsubscribe from the same group.

Fixes: 47c4b0de

 ("tools/lib/thermal: Add a thermal library")
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lore.kernel.org/r/20230202102812.453357-1-vincent.guittot@linaro.org
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>