Get rif of the protection against faulty inheritance of actors

The disadvantage is that then the child can access and modify parent's private members, such as mActivity, and cause rather unpredictable behaviours.
I don't think it's worth having a protection which in the end would cause a different kind of bugs.

Author: knopers8

Framework/include/QualityControl/Actor.h

@@ -123,12 +123,7 @@ class Actor
     return actor_helpers::requiresService<ConcreteActor, S>();
   }
 
-  // a trick to prevent bugs like "class TaskRunner : public Actor<CheckRunner>"
-  // by keeping the constructor private, thus allowing only CheckRunner to initialize Actor<CheckRunner>
-  // see https://www.fluentcpp.com/2017/05/12/curiously-recurring-template-pattern/
-  friend ConcreteActor;
-
- private:
+ public:
   explicit Actor(const ServicesConfig& servicesConfig)
     : mServicesConfig{ servicesConfig },
       mActivity{ servicesConfig.activity }