Remove NO_THREADS fallback code, Godot 4 requires thread support

This also removes `OS::can_use_threads` from the public API since it's always
true.

drivers/unix/os_unix.cpp

@@ -126,9 +126,7 @@ int OS_Unix::unix_initialize_audio(int p_audio_driver) {
 }
 
 void OS_Unix::initialize_core() {
-#if !defined(NO_THREADS)
 	init_thread_posix();
-#endif
 
 	FileAccess::make_default<FileAccessUnix>(FileAccess::ACCESS_RESOURCES);
 	FileAccess::make_default<FileAccessUnix>(FileAccess::ACCESS_USERDATA);