Remove protos::CreateMessage, protos::ClearExtension both unused

Update ExtensionNotFoundError from protos to hpb

PiperOrigin-RevId: 665855910

protos/protos.h

@@ -26,12 +26,9 @@ using hpb::internal::Serialize;
 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;