Remove unused ISingleton.h

GUI/Models/ISingleton.h

-//  ************************************************************************************************
-//
-//  BornAgain: simulate and fit reflection and scattering
-//
-//! @file      GUI/Models/ISingleton.h
-//! @brief     Defines the standard mix-in ISingleton.
-//!
-//! @homepage  http://www.bornagainproject.org
-//! @license   GNU General Public License v3 or higher (see COPYING)
-//! @copyright Forschungszentrum Jülich GmbH 2018
-//! @authors   Scientific Computing Group at MLZ (see CITATION, AUTHORS)
-//
-//  ************************************************************************************************
-
-#ifndef BORNAGAIN_GUI_MODELS_ISINGLETON_H
-#define BORNAGAIN_GUI_MODELS_ISINGLETON_H
-
-//! Base class for singletons.
-//! @ingroup tools_internal
-
-template <class T> class ISingleton {
-public:
-    static T& instance()
-    {
-        static T m_instance;
-        return m_instance;
-    }
-
-    ISingleton(const ISingleton&) = delete;
-    ISingleton& operator=(const ISingleton&) = delete;
-    ISingleton(ISingleton&&) = delete;
-    ISingleton& operator=(ISingleton&&) = delete;
-    
-protected:
-    ISingleton() = default;
-    ~ISingleton() = default;
-};
-
-#endif // BORNAGAIN_GUI_MODELS_ISINGLETON_H