test: remove common.isOSXMojave

common.isOSXMojave was added because it was believed that there was a
bug in macOS Mojave that allowed unprivileged users to bind to
privileged ports. As it turns out, that was a feature not a bug. It is
likely to be in all future versions of macOS. Remove isOSXMojave and
skip appropriate tests based on isOSX.

Refs: https://news.ycombinator.com/item?id=18302380
PR-URL: https://github.com/nodejs/node/pull/25658


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>