Remove dummy int64 field in protobuf format

message.proto
```proto
message FileSystemTestMessage {
// TODO: We have to add a dummy field with type int64 at the beginning,
// otherwise exception will be thrown.
// https://issues.apache.org/jira/browse/FLINK-31944
```

```proto
message AllTypesTest {
// TODO: We have to add a dummy field with type int64 at the beginning,
// otherwise exception will be thrown.
// https://issues.apache.org/jira/browse/FLINK-31944
```

Start in message.proto by locating the FileSystemTestMessage and AllTypesTest definitions and reviewing why the dummy int64 fields were added. Check FLINK-31944 for the relevant compatibility or exception details, then identify the existing tests for these protobuf messages. Done means the dummy fields can be removed without the exception returning and the affected tests still pass.

Written by the indexing model from the issue text.