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?

[dwnusbaum]

#388 now has a test that requires GCFlushedOutputStream, so we can at least verify that it works as intended under specialized conditions. I'm closing this in favor of that PR for now. However, if that PR turns out to cause any problems, then we should just delete the whole class instead.