Libraries: Use default constructors/destructors in LibJS

https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#cother-other-default-operation-rules

"The compiler is more likely to get the default semantics right and
you cannot implement these functions better than the compiler."

Userland/Libraries/LibJS/Runtime/StringObject.cpp

@@ -26,10 +26,6 @@ StringObject::StringObject(PrimitiveString& string, Object& prototype)
 {
 }
 
-StringObject::~StringObject()
-{
-}
-
 void StringObject::initialize(GlobalObject& global_object)
 {
     auto& vm = this->vm();