Drop frozen legacy examples (SDK 45 / React 17)

Remove four example directories that no longer serve a useful purpose
and have only been receiving dependabot bumps since 2022:

- examples/cookbook-expo (Expo SDK 45 / React 17, frozen)
- examples/cookbook-rn (bare RN of the same vintage)
- examples/cookbook-rn-shared (40 sources only consumed by the two above)
- examples/expo-gl-react-camera-effects (predecessor of
  packages/cookbook-expo-camera, replaced upstream)

examples/next-js and packages/cookbook are intentionally kept; the
historical code remains accessible via git history.

Also remove the now-stale `examples/*/shared/` rule from .gitignore,
which existed for the cookbook-rn-shared symlink layout.

Author: greweb

.gitignore

@@ -12,7 +12,6 @@ packages/gl-react/gl-react.js
 packages/gl-react-dom/gl-react-dom.js
 packages/tests/node_modules_to_test/
 packages/cookbook/src/API.json
-examples/*/shared/
 .yarn
 .pnp.cjs
 .pnp.loader.mjs