LibJS: Remove Object::value_of()

Being really close to Object.prototype.valueOf() name wise makes this
unnecessarily confusing - while it sometimes serves as the
implementation of a valueOf() function, it's an abstraction which the
spec doesn't have.
Use the appropriate getters to retrieve specific internal slots instead,
most commonly [[FooData]] from the primitive wrapper objects.
For the Object class specifically, use the Value(Object*) ctor instead.

Userland/Libraries/LibJS/Runtime/Object.h

@@ -167,7 +167,6 @@ public:
 
     virtual const char* class_name() const override { return "Object"; }
     virtual void visit_edges(Cell::Visitor&) override;
-    virtual Value value_of() const { return Value(const_cast<Object*>(this)); }
 
     Value get_direct(size_t index) const { return m_storage[index]; }