Does Flink ever delete any sink S3 files?

[Dan Hill]

Hi.

I'm debugging an issue with a system that listens for files written to S3.
I'm assuming Flink does not modify sink objects after they've been
written.  I've seen a minicluster test locally write a ".part-" file.  I
wanted to double check to make sure S3 sinks won't write and delete files.

```
FileSink.forBulkFormat(new Path(...), factory)
        .withBucketAssigner(new DateHourBucketAssigner<...>(...))
        .withRollingPolicy(OnCheckpointRollingPolicy.build())
        .build())
    .uid("sink-s3")
    .name("S3 sink");
```