Delete GCFlushedOutputStream

[dwnusbaum]

See #83 (comment). Alternative to #388.

Testing done

Existing tests here pass for me locally, but if we want to go through with this we need to run it through the PCT.

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[jglick]

Prefer #388 but I only have a dim memory of what this was (supposed to be) solving. Perhaps remote TaskListeners which never get flushed before the callable exits?