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/FunctionEnvironment.cpp

@@ -16,10 +16,6 @@ FunctionEnvironment::FunctionEnvironment(Environment* parent_scope)
 {
 }
 
-FunctionEnvironment::~FunctionEnvironment()
-{
-}
-
 void FunctionEnvironment::visit_edges(Visitor& visitor)
 {
     Base::visit_edges(visitor);