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/ios/detect.py

@@ -55,7 +55,7 @@ def configure(env):
     ## Build type
 
     if env["target"].startswith("release"):
-        env.Append(CPPDEFINES=["NDEBUG", ("NS_BLOCK_ASSERTIONS", 1)])
+        env.Append(CPPDEFINES=[("NS_BLOCK_ASSERTIONS", 1)])
         if env["optimize"] == "speed":  # optimize for speed (default)
             # `-O2` is more friendly to debuggers than `-O3`, leading to better crash backtraces
             # when using `target=release_debug`.
@@ -67,8 +67,7 @@ def configure(env):
             env.Append(LINKFLAGS=["-Os"])
 
     elif env["target"] == "debug":
-        env.Append(CCFLAGS=["-gdwarf-2", "-O0"])
-        env.Append(CPPDEFINES=["_DEBUG", ("DEBUG", 1)])
+        env.Append(CCFLAGS=["-g", "-O0"])
 
     ## LTO