remove useless ?s (clippy::needless_question_marks)

Example code:
```
fn opts() -> Option<String> {
    let s: Option<String> = Some(String::new());
    Some(s?) // this can just be "s"
}
```

compiler/rustc_infer/src/infer/canonical/query_response.rs

@@ -507,12 +507,7 @@ impl<'cx, 'tcx> InferCtxt<'cx, 'tcx> {
 
         // Unify the original value for each variable with the value
         // taken from `query_response` (after applying `result_subst`).
-        Ok(self.unify_canonical_vars(
-            cause,
-            param_env,
-            original_values,
-            substituted_query_response,
-        )?)
+        self.unify_canonical_vars(cause, param_env, original_values, substituted_query_response)
     }
 
     /// Converts the region constraints resulting from a query into an