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.cpp

@@ -49,9 +49,4 @@ RegExpObject::~RegExpObject()
 {
 }
 
-Value RegExpObject::to_string() const
-{
-    return js_string(heap(), String::formatted("/{}/{}", content(), flags()));
-}
-
 }