chore: remove deprecated sample

[glasnt]

Description

Fixes lint warnings on all PRs (including #816)

b/407443927 cl/742137022

• [cx] Yes, merge this PR after it is approved

[snippet-bot[bot]]

Here is the summary of changes.

You are about to delete 1 region tag.

• vertex_ai/user_managed_notebooks_instance/main.tf:18, tag aiplatform_create_user_managed_notebooks_instance_sample

---

This comment is generated by snippet-bot. If you find problems with this result, please file an issue at: https://github.com/googleapis/repo-automation-bots/issues. To update this comment, add snippet-bot:force-run label or use the checkbox below:

• [ ] Refresh this comment