LibJS: Implement RegExp.prototype.toString() as standalone function

This should not just inherit Object.prototype.toString() (and override
Object::to_string()) but be its own function, i.e.
'RegExp.prototype.toString !== Object.prototype.toString'.

Libraries/LibJS/Runtime/RegExpObject.h

@@ -43,8 +43,6 @@ public:
     const String& content() const { return m_content; }
     const String& flags() const { return m_flags; }
 
-    Value to_string() const override;
-
 private:
     virtual bool is_regexp_object() const override { return true; }