chore: delete broken examples (#404)

<!-- CURSOR_SUMMARY -->
> [!NOTE]
> **Low Risk**
> Documentation/example-only deletion with no changes to the published
SDK or runtime library code.
> 
> **Overview**
> Removes the **`examples/async-provider`** sample app entirely (source,
webpack/babel/eslint config, `package.json`, and README).
> 
> That example demonstrated **`asyncWithLDProvider`** with an Express +
hot-reload SPA using **`withLDConsumer`** and **`useFlags`** against a
`dev-test-flag`; it is no longer maintained in the repo.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
c60ab7f. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: joker23