control_allocator: change SequentialDesaturation to existing MC mixer

And limit the operations to the number of configured outputs.

Only using the number of configured actuators reduces CPU load by ~2% on
F7 @1khz.

src/modules/control_allocator/ActuatorEffectiveness/ActuatorEffectivenessStandardVTOL.hpp

@@ -58,6 +58,7 @@ public:
 	 */
 	void setFlightPhase(const FlightPhase &flight_phase) override;
 
+	int numActuators() const override { return 7; }
 protected:
 	bool _updated{true};
 };