SCons: Cleanup DEBUG, _DEBUG and NDEBUG defines

- `_DEBUG` is MSVC specific so it didn't make much sense to define for
  Android and iOS builds.
- iOS was the only platform to define `DEBUG`. We don't use it anywhere
  outside thirdparty code, which we usually don't intend to debug, so it
  seems better to be consistent with other platforms.
- Consistently define `NDEBUG` to disable assert behavior in both `release`
  and `release_debug` targets. This used to be set for `release` for all
  platforms, and `release_debug` for Android and iOS only.
- Due to the above, I removed the only use we made of `assert()` in Godot
  code, which was only implemented for Unix anyway, should have been
  `DEV_ENABLED`, and is in PoolAllocator which we don't actually use.
- The denoise and recast modules keep defining `NDEBUG` even for the `debug`
  target as we don't want OIDN and Embree asserting all over the place.

drivers/unix/os_unix.cpp

@@ -51,7 +51,6 @@
 #include <sys/sysctl.h>
 #endif
 
-#include <assert.h>
 #include <dlfcn.h>
 #include <errno.h>
 #include <poll.h>
@@ -104,10 +103,6 @@ static void _setup_clock() {
 }
 #endif
 
-void OS_Unix::debug_break() {
-	assert(false);
-}
-
 static void handle_interrupt(int sig) {
 	if (!EngineDebugger::is_active()) {
 		return;