Remove back-port of C++17 features

[bartgol]

We implemented some C++17 features when we were still stuck with C++14, like ekat::any or ekat::enable_shared_from_this. Since ekat now requires C++17, all these features are available from the std library, so we should remove the hand-crafted classes. We can do a deprecation first, by doing something like

#ifndef EKAT_STD_ANY_HPP
#define EKAT_STD_ANY_HPP

#pragma warning("Warning: Including ekat_std_any.hpp is discouraged. Please avoid using ekat::any, and rely on std::any instead.")
namespace ekat {

...

}

#endif // EKAT_STD_ANY_HPP