Revert "Show turbofish optional"

This reverts commit 8b19147.

src/error.rs

@@ -577,7 +577,7 @@ where
         // Caller is looking for an E pointer and e is ErrorImpl<E>, take a
         // pointer to its E field.
         let unerased = e.cast::<ErrorImpl<E>>().deref();
-        Some(Ref::new(&unerased._object).cast())
+        Some(Ref::new(&unerased._object).cast::<()>())
     } else {
         None
     }