Remove the outdated draft document

[hayatoito]

As README.md explains, Web Bundles format is now under development in wpack-wg/bundled-responses reposiotry.

Let's remove the outdated draft document in this repository, and update the links to that [1] with the latest draft [2].

• 1: https://wicg.github.io/webpackage/draft-yasskin-wpack-bundled-exchanges.html.
• 2: https://wpack-wg.github.io/bundled-responses/draft-ietf-wpack-bundled-responses.html.

[hayatoito]

Thanks for the review. I've rebased the PR on main. Let me merge the PR.