test: remove test-timers-first-fire

According to the documentation, "io.js makes no guarantees
about the exact timing of when the callback will fire"
(https://iojs.org/api/timers.html#timers_settimeout_callback_delay_arg),
hence this test is not necessary.

PR-URL: #2458
Reviewed-By: Ben Noordhuis <ben@strongloop.com>
Reviewed-By: Alexis Campailla <alexis@janeasystems.com>