Support GCC 12 C++20 mode

[karjonas]

This fix just removes a few templates which is needed since C++20 breaks previously valid code. See https://timsong-cpp.github.io/cppwp/n4861/diff.cpp17.class

This is basically what is allowed and not in c++20:

template<class T>
struct A {
  A<T>();  // error: simple-template-id not allowed for constructor
  A(int);  // OK, injected-class-name used
  ~A<T>(); // error: simple-template-id not allowed for destructor
};