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_trait_selection/src/traits/query/type_op/custom.rs

@@ -43,7 +43,7 @@ where
             info!("fully_perform({:?})", self);
         }
 
-        scrape_region_constraints(infcx, || Ok((self.closure)(infcx)?))
+        scrape_region_constraints(infcx, || (self.closure)(infcx))
     }
 }