Remove own smart pointers

[Kaffeine]

Remove own smart pointer classes or rationalize their existance.

• Tp::WeakPtr
• Tp::SharedPtr
• Tp::RefCounted

[z3ntu]

Also causes some compile warnings when compiling with C++17 for example

/usr/include/telepathy-qt5/TelepathyQt/shared-ptr.h: In constructor 'Tp::SharedPtr<T>::SharedPtr(const Tp::WeakPtr<T>&)':
/usr/include/telepathy-qt5/TelepathyQt/shared-ptr.h:112:26: warning: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
  112 |             register int tmp = sc->strongref.fetchAndAddOrdered(0);
      |                          ^~~