test: fix flaky test-net-GH-5504

The test is failing on `SmartOS` quite often. Removing the timeout seems
to fix it.

Fixes: https://github.com/nodejs/node/issues/8930
PR-URL: https://github.com/nodejs/node/pull/9461


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>