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.

modules/raycast/SCsub

@@ -63,7 +63,8 @@ if env["builtin_embree"]:
     thirdparty_sources = [thirdparty_dir + file for file in embree_src]
 
     env_raycast.Prepend(CPPPATH=[thirdparty_dir, thirdparty_dir + "include"])
-    env_raycast.Append(CPPDEFINES=["EMBREE_TARGET_SSE2", "EMBREE_LOWEST_ISA", "TASKING_INTERNAL", "NDEBUG"])
+    env_raycast.Append(CPPDEFINES=["EMBREE_TARGET_SSE2", "EMBREE_LOWEST_ISA", "TASKING_INTERNAL"])
+    env_raycast.AppendUnique(CPPDEFINES=["NDEBUG"])  # No assert() even in debug builds.
 
     if not env.msvc:
         if env["arch"] == "x86_64":