Moves this from doom-ui to doom-start, since there is more savings to be
had if this is done early.
Also moves the menu-bar fix for macos out of the :os macos module into
doom-start, because it is a fix (and for a Doom optimization) and not a
feature, so it shouldn't be behind a module.
Some users may not be using a theme (*shudder*), in which case
ns-auto-titlebar-mode never kicks in.
That said, this "fix" didn't fix the targeted issue anyhow.
If triggered too early, it may read the wrong frame background mode and
you'll get dark text on a dark titlebar in MacOS dark mode (or light
text on a light titlebar in light mode).
