src: remove unused typedef

Seems to have been overlooked in commit dd93c53c ("Make node::DLOpen use
uv_dlopen") from 2011.

PR-URL: https://github.com/nodejs/node/pull/11322


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Nikolai Vavilov <vvnicholas@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>