thermal: core: Drop thermal_zone_device_exec()

Because thermal_zone_device_exec() has no users any more and there are
no plans to use it anywhere, revert commit 9a99a996 ("thermal: core:
Introduce thermal_zone_device_exec()") that introduced it.

No functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>