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);

drivers/unix/thread_posix.cpp

@@ -28,7 +28,7 @@
 /* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.                */
 /*************************************************************************/
 
-#if (defined(UNIX_ENABLED) || defined(PTHREAD_ENABLED)) && !defined(NO_THREADS)
+#if defined(UNIX_ENABLED) || defined(PTHREAD_ENABLED)
 
 #include "thread_posix.h"
 
@@ -73,4 +73,4 @@ void init_thread_posix() {
 	Thread::_set_platform_funcs(&set_name, nullptr);
 }
 
-#endif
+#endif // UNIX_ENABLED || PTHREAD_ENABLED

drivers/unix/thread_posix.h

@@ -31,8 +31,6 @@
 #ifndef THREAD_POSIX_H
 #define THREAD_POSIX_H
 
-#if !defined(NO_THREADS)
 void init_thread_posix();
-#endif
 
 #endif // THREAD_POSIX_H