Remove obsolete comment

Author: celinval

compiler/stable_mir/src/ty.rs

@@ -226,7 +226,6 @@ pub struct ImplDef(pub DefId);
 pub struct RegionDef(pub DefId);
 
 /// A list of generic arguments.
-/// The second field is for internal usage to allow retrieving the internal representation.
 #[derive(Clone, Debug)]
 pub struct GenericArgs(pub Vec<GenericArgKind>);