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.

platform/windows/detect.py

@@ -350,7 +350,6 @@ def configure_msvc(env, vcvars_msvc_config):
 
     elif env["target"] == "debug":
         env.AppendUnique(CCFLAGS=["/Zi", "/FS", "/Od", "/EHsc"])
-        # Allow big objects. Only needed for debug, see MinGW branch for rationale.
         env.Append(LINKFLAGS=["/DEBUG"])
 
     if env["debug_symbols"]: