Move WeakRef to CoreBind, since it shouldn't be (and isn't) used internally

[Ivorforce]

WeakRef is a way to store a weak reference to an Object. It's implemented as a RefCounted subtype, because it can be useful for GDScript users (and otherwise).

However, as an Object subtype, it's also unnecessarily slow and wasteful. We shouldn't use it internally — notably, we currently don't use it internally at all. If we need a weak Object reference, we should be using something more performant. This 'something' could be proposed in a future PR, if it is needed (it would probably be a templated ObjectID wrapper).

Since there is no user of this class in the engine, it's trivial to move it to CoreBind. It was especially wrong to have it in ref_counted.h since it is usable not just for RefCounted, but for any Object subclass.