test: remove arbitrary timer

`test-dgram-send-callback-buffer-length` was timing out (via the
200ms timeout in the code) on FreeBSD in CI. The 200ms timeout is
arbitrary and not necessary. Remove it.

PR-URL: https://github.com/nodejs/node/pull/9197


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>