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/WeakRef.cpp

@@ -21,10 +21,6 @@ WeakRef::WeakRef(Object* object, Object& prototype)
 {
 }
 
-WeakRef::~WeakRef()
-{
-}
-
 void WeakRef::remove_dead_cells(Badge<Heap>)
 {
     VERIFY(m_value);