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.

SConstruct

@@ -386,6 +386,9 @@ if env_base["target"] == "debug":
     # DEV_ENABLED enables *engine developer* code which should only be compiled for those
     # working on the engine itself.
     env_base.Append(CPPDEFINES=["DEV_ENABLED"])
+else:
+    # Disable assert() for production targets (only used in thirdparty code).
+    env_base.Append(CPPDEFINES=["NDEBUG"])
 
 # SCons speed optimization controlled by the `fast_unsafe` option, which provide
 # more than 10 s speed up for incremental rebuilds.
@@ -669,7 +672,6 @@ if selected_platform in platform_list:
             print("       Use `tools=no target=release` to build a release export template.")
             Exit(255)
         suffix += ".opt"
-        env.Append(CPPDEFINES=["NDEBUG"])
     elif env["target"] == "release_debug":
         if env["tools"]:
             suffix += ".opt.tools"