Remove protos::CreateMessage, protos::ClearExtension both unused
Update ExtensionNotFoundError from protos to hpb

PiperOrigin-RevId: 665855910

protos/protos.h

diff --git a/protos/protos.h b/protos/protos.h
index ffd849b..ca01c1e 100644
--- a/protos/protos.h
+++ b/protos/protos.h
@@ -26,12 +26,9 @@
 using hpb::internal::SetExtension;
 using hpb::interop::upb::GetMiniTable;
 }  // namespace internal
-using hpb::CreateMessage;
 using hpb::Parse;
 using hpb::Serialize;
 
-using hpb::ClearExtension;
-using hpb::ExtensionNotFoundError;
 using hpb::ExtensionNumber;
 using hpb::GetExtension;
 using hpb::HasExtension;