Remove x-pack/docs dir (elastic#99304)

**Problem:** With elastic#99209, the elasticsearch repo's `x-pack/docs` directory no longer contains docs for the Elasticsearch Guide in 8.10+.

The `gradle.build` files in the directory were only left in so that the docs build doesn't break. That's been fixed with elastic/docs#2753.

**Solution:** Removes the `x-pack/docs` directory.

Depends on elastic#99209, elastic/docs#2753
Closes: https://github.com/elastic/platform-docs-team/issues/208