AK+WindowServer: Remove did_construct() framework used only once

There is also make_ref_counted(), which does not call did_construct(),
so the method was not guaranteed to be run. Since there is only a single
user, and `WindowServer::Window` is a final class anyway (so there is no
need to separate the constructor and post-constructor phases), let's get
rid of this concept.

(The following commits reduce the opportunities to call
make_ref_counted, but still.)

Userland/Libraries/LibCore/Object.h

@@ -70,8 +70,6 @@ public:                                                                \
     static inline NonnullRefPtr<klass> construct(Args&&... args)       \
     {                                                                  \
         auto obj = adopt_ref(*new Klass(forward<Args>(args)...));      \
-        if constexpr (requires { declval<Klass>().did_construct(); })  \
-            obj->did_construct();                                      \
         return obj;                                                    \
     }