Remove godot-headers submodule, copy files directly

With the new GDExtension API, the headers are just two files, one of
which is generated and needs to always be kept in sync with the Godot
engine version.

So there's little practical use for using godot-headers as a submodule
anymore, and it only makes godot-cpp updates more cumbersome.

Custom headers (i.e. a custom API JSON) can still be used by passing
the `headers_dir` SCons option.

godot-headers/README.md

@@ -0,0 +1,16 @@
+# godot-headers
+
+This repository contains C headers for
+[**Godot Engine**](https://github.com/godotengine/godot)'s *GDNative Extensions* API.
+
+## Updating Headers
+
+If the current branch is not up-to-date for your needs, or if you want to sync
+the headers with your own modified version of Godot, here is the update
+procedure used to sync this repository with upstream releases:
+
+- Compile [Godot Engine](https://github.com/godotengine/godot) at the specific
+  version/commit which you are using.
+- Use the compiled executable to generate the `extension_api.json` file with:
+  `godot --dump-extension-api extension_api.json`
+- Copy the file `core/extension/gdnative_interface.h` to `godot`