Fix `SerializedMessage{,Backward}Writer` build `#if !__cpp_concepts`:
remove defaulted template parameter from definition which was already present
in the declaration.

PiperOrigin-RevId: 896383147

riegeli/messages/serialized_message_backward_writer.h

diff --git a/riegeli/messages/serialized_message_backward_writer.h b/riegeli/messages/serialized_message_backward_writer.h
index f2bc44e..3da4e4e 100644
--- a/riegeli/messages/serialized_message_backward_writer.h
+++ b/riegeli/messages/serialized_message_backward_writer.h
@@ -480,7 +480,7 @@
                   IsStringifiable<Values...>,
                   std::negation<std::is_convertible<
                       Values&&, const google::protobuf::MessageLite&>>...>,
-              int> = 0
+              int>
 #endif
           >
 #if __cpp_concepts

riegeli/messages/serialized_message_writer.h

diff --git a/riegeli/messages/serialized_message_writer.h b/riegeli/messages/serialized_message_writer.h
index 5a9ff89..fea8f1a 100644
--- a/riegeli/messages/serialized_message_writer.h
+++ b/riegeli/messages/serialized_message_writer.h
@@ -583,7 +583,7 @@
                   IsStringifiable<Values...>,
                   std::negation<std::is_convertible<
                       Values&&, const google::protobuf::MessageLite&>>...>,
-              int> = 0
+              int>
 #endif
           >
 #if __cpp_concepts
@@ -823,7 +823,7 @@
                   IsStringifiable<Values...>,
                   std::negation<std::is_convertible<
                       Values&&, const google::protobuf::MessageLite&>>...>,
-              int> = 0
+              int>
 #endif
           >
 #if __cpp_concepts