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/NumberObject.h

@@ -19,8 +19,6 @@ public:
     NumberObject(double, Object& prototype);
     virtual ~NumberObject() override;
 
-    virtual Value value_of() const override { return Value(m_value); }
-
     double number() const { return m_value; }
 
 private: