Summary: Fixes T7620. I don't fully understand exactly what's going on here, but we don't actually need to call `flush()`. Test Plan: - Put timing code around the `echo`. - Made a fake page that emitted a lot of data. - Saw the `echo` block proportionate to data size under `curl --limit-rate ...`. - See T7620. - Downloaded a large file, got a reasonable progress bar and no obvious memory use issues. Reviewers: btrahan Reviewed By: btrahan Subscribers: jlarouche, rbalik, epriestley Maniphest Tasks: T7620 Differential Revision: https://secure.phabricator.com/D12127