Don't call flush() when emitting responses

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