ci: remove the release-chain rehearsal workflow

Run 34260211236 on the pull request called pypi-publish.yml through
workflow_call aimed at TestPyPI: the trusted-publishing exchange succeeded,
attestations were generated, and both archives were uploaded (already
present, so skipped). Same-repository reusable workflows pass PyPI's
job_workflow_ref check, which names the called file.

.github/workflows/tmp-release-chain-rehearsal.yml

@@ -1,22 +0,0 @@
-name: TEMPORARY release-chain rehearsal (remove before merge)
-
-# Exercises the exact mechanism backend-release uses to publish: a same-repo
-# reusable-workflow call of pypi-publish.yml with trusted publishing, aimed at
-# TestPyPI. PyPI verifies the job_workflow_ref claim against the registered
-# filename, which for a reusable workflow is the called file, so this run
-# either proves the call path or shows the publisher error before 0.20.0.
-
-on:
-  pull_request:
-    branches: [main]
-
-permissions: {}
-
-jobs:
-  pypi-testpypi:
-    uses: $/.github/workflows/pypi-publish.yml
-    with:
-      target: testpypi
-    permissions:
-      contents: read
-      id-token: write