nit: use let-chain in Cfg::simplify_with

Author: poliorcetics

src/librustdoc/clean/cfg.rs

@@ -241,10 +241,10 @@ impl Cfg {
                 1 => sub_cfgs.pop(),
                 _ => Some(Cfg::All(sub_cfgs)),
             };
-        } else if let Cfg::All(b) = assume {
-            if b.contains(self) {
-                return None;
-            }
+        } else if let Cfg::All(b) = assume
+            && b.contains(self)
+        {
+            return None;
         }
 
         Some(self.clone())