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/Forward.h

@@ -166,8 +166,6 @@ JS_ENUMERATE_ERROR_SUBCLASSES
 JS_ENUMERATE_TYPED_ARRAYS
 #undef __JS_ENUMERATE
 
-struct Argument;
-
 template<class T>
 class Handle;