test: unmark test-http-regr-gh-2928 as flaky

The flakiness issue for test-http-regr-gh-2928 on SmartOS was resolved
in late February in https://github.com/nodejs/node/pull/5454. This
change removes its flaky designation in sequential.status.

PR-URL: https://github.com/nodejs/node/pull/6540


Reviewed-By: James M Snell <jasnell@gmail.com>