CI: Fix dumping GDExtension interface and API for godot-cpp

Follow-up to https://github.com/godotengine/godot-cpp/pull/960.

Fix exit code for --dump-extension-api and --dump-gdextension-interface.

Removed the planned API validation step as we still didn't implement
anything, and maintaining a stub isn't useful.

misc/scripts/compare_extension_api.py

@@ -1,11 +0,0 @@
-#!/usr/bin/env python
-# -*- coding: utf-8 -*-
-
-import sys
-
-# TODO:
-# Add a process that compares the original godot-cpp/godot-headers/extension_api.json with the new extension_api.json (both passed as arguments) and reports any API calls that have been removed.
-# If we only have additions or no changes to the file, we pass
-# For now we deem this too early because the API isn't stable enough yet.
-
-sys.exit(0)