thermal: core: Use the new 'thermal_zone_destroy_device_groups()' helper function

Simplify code by using the new 'thermal_zone_destroy_device_groups()'
helper function.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>