Revert "Metadata handling rewrite"
diff --git a/src/cpp/client/secure_credentials.h b/src/cpp/client/secure_credentials.h
index 713654a..281db17 100644
--- a/src/cpp/client/secure_credentials.h
+++ b/src/cpp/client/secure_credentials.h
@@ -70,7 +70,7 @@
   grpc_call_credentials* const c_creds_;
 };
 
-class MetadataCredentialsPluginWrapper final : private GrpcLibraryCodegen {
+class MetadataCredentialsPluginWrapper final {
  public:
   static void Destroy(void* wrapper);
   static void GetMetadata(void* wrapper, grpc_auth_metadata_context context,