Update test for new serde

Author: kjeremy

crates/ra_proc_macro_srv/src/tests/fixtures/test_serialize_proc_macro.txt

@@ -14,7 +14,7 @@ SUBTREE $
       PUNCH   , [alone] 4294967295
       IDENT   unused_qualifications 4294967295
   IDENT   const 4294967295
-  IDENT   _IMPL_SERIALIZE_FOR_Foo 4294967295
+  IDENT   _ 4294967295
   PUNCH   : [alone] 4294967295
   SUBTREE () 4294967295
   PUNCH   = [alone] 4294967295