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>

test/sequential/sequential.status

@@ -13,7 +13,6 @@ prefix sequential
 [$system==macos]
 
 [$system==solaris] # Also applies to SmartOS
-test-http-regr-gh-2928      : PASS,FLAKY
 
 [$system==freebsd]