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/FinalizationRegistry.cpp

@@ -17,10 +17,6 @@ FinalizationRegistry::FinalizationRegistry(Realm& realm, JS::JobCallback cleanup
 {
 }
 
-FinalizationRegistry::~FinalizationRegistry()
-{
-}
-
 void FinalizationRegistry::add_finalization_record(Cell& target, Value held_value, Object* unregister_token)
 {
     VERIFY(!held_value.is_empty());