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/Symbol.cpp

@@ -16,10 +16,6 @@ Symbol::Symbol(Optional<String> description, bool is_global)
 {
 }
 
-Symbol::~Symbol()
-{
-}
-
 Symbol* js_symbol(Heap& heap, Optional<String> description, bool is_global)
 {
     return heap.allocate_without_global_object<Symbol>(move(description), is_global);