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/MapIterator.cpp

@@ -22,10 +22,6 @@ MapIterator::MapIterator(Map& map, Object::PropertyKind iteration_kind, Object&
 {
 }
 
-MapIterator::~MapIterator()
-{
-}
-
 void MapIterator::visit_edges(Cell::Visitor& visitor)
 {
     Base::visit_edges(visitor);