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/BigIntObject.h

@@ -23,11 +23,6 @@ public:
     BigInt const& bigint() const { return m_bigint; }
     BigInt& bigint() { return m_bigint; }
 
-    virtual Value value_of() const override
-    {
-        return Value(&m_bigint);
-    }
-
 private:
     virtual void visit_edges(Visitor&) override;