remove experimental/memory work-around

This was an attempt to handle pre-C++-17, but it would need
work for make_shared.  In any case, it isn't available on VS2020 etc
apparently.

src/include/stir/shared_ptr.h

@@ -35,21 +35,10 @@ using boost::static_pointer_cast;
 #  define MAKE_SHARED boost::make_shared
 } // namespace stir
 #else
-#  if __cplusplus < 201703L
-#    include <experimental/memory>
-namespace stir
-{
-using std::experimental::shared_ptr;
-}
-#  else
-#    include <memory>
+
 namespace stir
 {
 using std::shared_ptr;
-}
-#  endif
-namespace stir
-{
 using std::dynamic_pointer_cast;
 using std::static_pointer_cast;
 //! work-around for using std::make_shared on old compilers