src: remove superfluous HandleScope

Accessors implicitly run inside a HandleScope, UDPWrap::GetFD() doesn't
need to create one explicitly.

PR-URL: https://github.com/nodejs/node/pull/16482


Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Anna Henningsen <anna@addaleax.net>