comment out record of now-unused error code E0563

The sole appearance of this code was deleted in 6383de1; the existing practice
in these cases seems to be to comment out its mention in
`register_diagnostics!`.

src/librustc_typeck/diagnostics.rs

@@ -4709,7 +4709,7 @@ register_diagnostics! {
            // between structures with the same definition
     E0521, // redundant default implementations of trait
     E0533, // `{}` does not name a unit variant, unit struct or a constant
-    E0563, // cannot determine a type for this `impl Trait`: {}
+//  E0563, // cannot determine a type for this `impl Trait`: {} // removed in 6383de15
     E0564, // only named lifetimes are allowed in `impl Trait`,
            // but `{}` was found in the type `{}`
     E0567, // auto traits can not have type parameters