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.

(cherry picked from commit 26e9145c26)

modules/denoise/SCsub

@@ -103,9 +103,9 @@ env_oidn.Append(
         "__STDC_LIMIT_MACROS",
         "DISABLE_VERBOSE",
         "MKLDNN_ENABLE_CONCURRENT_EXEC",
-        "NDEBUG",
     ]
 )
+env_oidn.AppendUnique(CPPDEFINES=["NDEBUG"])  # No assert() even in debug builds.
 
 env_thirdparty = env_oidn.Clone()
 env_thirdparty.disable_warnings()