Remove unused mHook attribute in Operator

mHook attribute previously introduced in Operator class was not used. Since then, a new Hook system has been implemented in Node.

Thus removing deprecated feature.

include/aidge/hook/ExecTime.hpp

-/**
- * \file execTime.hpp
- * \brief execTime structure
- * \version file 1.0.0
- * \date Creation 27 June 2023
- * \date 27 June 2023
- * \par ChangeLog
- * \par
- *  v1.0.0, 27 June 2023<br>
- *  - Initial version.
- * \author mn271187, ik243221
- * \copyright
- *  Copyright (c) 2023 CEA, LIST, Embedded Artificial Intelligence Laboratory. All
- *  rights reserved.
- */
-
-#ifndef execTime_H_
-#define execTime_H_
-
-#include "aidge/operator/Operator.hpp"
-#include "aidge/hook/Hook.hpp"
-#include <memory>
-#include <chrono>
-#include <vector>
-
-namespace Aidge {
-
-class ExecTime : public Hook {
-private:
-    std::vector<std::chrono::high_resolution_clock::time_point> registeredTimes = std::vector<std::chrono::high_resolution_clock::time_point>();
-public:
-    ExecTime(const std::shared_ptr<Operator> op) : Hook(op) {}
-    ~ExecTime() = default;
-
-    void call() override final {
-        registeredTimes.push_back(std::chrono::high_resolution_clock::now());
-    }
-
-    static std::shared_ptr<ExecTime> create(const std::shared_ptr<Operator> op)
-    {
-        return std::make_shared<ExecTime>(op);
-    }
-
-    std::vector<std::chrono::high_resolution_clock::time_point> getTimes() {
-        return  registeredTimes;
-    }
-
-    std::chrono::high_resolution_clock::time_point getTime(size_t idx) {
-        return registeredTimes[idx];
-    }
-
-};
-
-namespace {
-    static Registrar<Hook> registrarHook_ExecTime({"execution_time"}, Aidge::ExecTime::create);
-}
-}
-
-#endif /* execTime_H_ */
\ No newline at end of file