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/SymbolObject.h

@@ -26,11 +26,6 @@ public:
     String description() const { return m_symbol.description(); }
     bool is_global() const { return m_symbol.is_global(); }
 
-    virtual Value value_of() const override
-    {
-        return Value(&m_symbol);
-    }
-
 private:
     virtual void visit_edges(Visitor&) override;