Ignore thread models when compiling with NO_THREAD

The thread model option for physics (2D) and rendering (single-unsafe,
single-safe, multithread), was causing crashes/locks when set as
multithreaded and exported for a platform that does not support threads
(namely HTML5).

This commit ensures that when threads support is not available, that
option is ignored, and the equivalent of "single-unsafe" is always used
instead.

(cherry picked from commit f3c6ac1d71)

servers/physics_2d/physics_2d_server_sw.cpp

@@ -1455,7 +1455,11 @@ Physics2DServerSW::Physics2DServerSW() {
 	island_count = 0;
 	active_objects = 0;
 	collision_pairs = 0;
+#ifdef NO_THREADS
+	using_threads = false;
+#else
 	using_threads = int(ProjectSettings::get_singleton()->get("physics/2d/thread_model")) == 2;
+#endif
 	flushing_queries = false;
 };