LibJS: Get rid of Argument and ArgumentVector

This was used for a feature where you could pass a vector of arguments
to enter_scope(). Since that way of passing arguments was not GC-aware
(as vectors use C++ heap storage), let's avoid using it and make sure
everything that needs to stay alive is either on the stack or in traced
storage instead.

Libraries/LibJS/Runtime/VM.h

@@ -64,13 +64,6 @@ struct CallFrame {
     bool is_strict_mode { false };
 };
 
-struct Argument {
-    FlyString name;
-    Value value;
-};
-
-typedef Vector<Argument, 8> ArgumentVector;
-
 class VM : public RefCounted<VM> {
 public:
     static NonnullRefPtr<VM> create();