macos: Remove old NSApp workaround related code

Those are leftovers from before we had gst_macos_main() in place - acted as fail-safes when our (Cerbero-only) GLib patch wasn't present, but even then often didn't really work correctly. We don't need these workarounds anymore, since the recommended way of running things on macOS with gst_macos_main() handles all that NSApp/NSRunLoop stuff for us by wrapping our code with an NSApplication.

!3532 (merged) and cerbero!1023 (merged) for more context.

This code was still active (by default, since we didn't set the GSTREAMER_GLIB_COCOA_NSAPPLICATION flag anywhere), but I tested around and behaviour seems completely unchanged after the cleanup. Tested both via gst-launch/play and in a compiled app (tutorials).