test: fix flaky test-http-agent-keepalive

Remove timeout delay causing flakiness on SmartOS.

Fixes: https://github.com/nodejs/node/issues/4492
PR-URL: https://github.com/nodejs/node/pull/4524


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rod Vagg <r@va.gg>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>