[lang/c++] Make new LLVM version compile with unused variable (#34230)

LLVM is making the unused variable warning a default:
92023b1509
This will make grpc compile after this LLVM update.

(release notes: no)

src/core/lib/channel/connected_channel.cc

@@ -548,7 +548,7 @@ ArenaPromise<ServerMetadataHandle> MakeClientCallPromise(
         }
         if (!done_recv_messages) {
           auto p = recv_messages();
-          if (auto* r = p.value_if_ready()) {
+          if (p.ready()) {
             // NOTE: ignore errors here, they'll be collected in the
             // recv_trailing_metadata.
             done_recv_messages = true;