Fix unused_parens lint

These are left over from a series of unfortunate rustfmt interactions.
ee75e6c0e9 (diff-c052b162f01a664f8184ef1855d1f1cfL767-R735)
cc2558b0dc (diff-c052b162f01a664f8184ef1855d1f1cfL733-R738)
9f38ca032e (diff-c052b162f01a664f8184ef1855d1f1cfL779-R782)

serde_derive/src/ser.rs

@@ -870,9 +870,9 @@ fn serialize_struct_variant<'a>(
     }
 
     let struct_trait = match context {
-        StructVariant::ExternallyTagged { .. } => (StructTrait::SerializeStructVariant),
+        StructVariant::ExternallyTagged { .. } => StructTrait::SerializeStructVariant,
         StructVariant::InternallyTagged { .. } | StructVariant::Untagged => {
-            (StructTrait::SerializeStruct)
+            StructTrait::SerializeStruct
         }
     };