Remove the ad-hoc cooling override now that COOL handles it natively

TempControllerBase.cooling/set_cooling() forced y=0 in advance of a
Sud ramp-down step; redundant now that the FSM's own COOL state
detects and handles a negative diff itself. Removed from the
controller, SudTask's anticipatory detection (and the Step message's
now-unused "Cooling" field), and demo_sud.py's mirror of it.

gui: the "Cool down" status-bar indicator now reflects the
controller's actual State ("States.COOL") via the TempCtrl channel,
rather than Sud's removed anticipatory flag - renamed
sud_cooling/set_sud_cooling to tc_cooling/set_tc_cooling to match.

scripts/demos/sud/demo_sud.py

@@ -87,14 +87,6 @@ if __name__ == '__main__':
 			ramping = sud.state == SudState.RAMPING
 			phase = step['ramp'] if ramping and 'ramp' in step else step.get('hold', {})
 
-			# A ramp step whose target is below the current temperature can
-			# only be reached by passive cooling - force the heater off,
-			# mirroring tasks/sud.py's SudTask.
-			cooling = bool(ramping and 'ramp' in step and step['temperature'] < ctrl.get_theta_ist() - TEMP_REACHED_TOLERANCE)
-			ctrl.set_cooling(cooling)
-			if cooling:
-				print("  -> cooling down to {}".format(step['temperature']))
-
 			print(f"Step {step['number']}: {step['descr']}")
 			apply_plant_params(step)
 			if ramping and 'ramp' in step: