ci: delete all pull_request_target workflows (#3382)

Removes two GitHub Actions workflows that used the `pull_request_target` event:

| Workflow | Purpose |
|----------|---------|
| `ci-rtfd.yml` | ReadTheDocs preview link on PRs |
| `labeler.yml` | Label PRs by changed files |

`pull_request_target` runs with full repo secrets and write permissions even for fork PRs — this is the exact vector exploited in the TanStack npm supply chain attack. Files are fully recoverable from git history if they need to be reimplemented safely later.

Author: bhimrazy