Hide GTSetupThreads, never shut them down

We've noticed crashes within `-[NSApplication terminate:]` where `GTShutdownThreads` will be run automatically and interfere with libgit2 threads that are still active. This can be avoided by just not shutting down threads.

It's unnecessary in most applications, and very error-prone due to its manipulation of global state. There's no reason to do it when the app is just exiting anyways.