Remove legacy tag-triggered release workflow (#1402)

NO_CHANGELOG=true

## Summary

Releases for `databricks-sdk-py` are now owned by the **secure public
registry releases pipeline** in
[`databricks/secure-public-registry-releases-eng`](https://github.com/databricks/secure-public-registry-releases-eng).
That pipeline calls `tagging.yml` in this repo to create the tag and
publish the artifact through the hardened path. The tag-push-triggered
workflow and the manual TestPyPI helper in this repo are now obsolete
entry points and are removed here.

## Per-file decisions

| File | Decision | Reason |
| --- | --- | --- |
| `.github/workflows/release.yml` | **Deleted** | Triggered only on
`push: tags: 'v*'`. Built the wheel, published to PyPI, and created the
GitHub release. Superseded by the secure release pipeline. |
| `.github/workflows/release-test.yml` | **Deleted** | Manual
`workflow_dispatch` helper that published to TestPyPI. Legacy companion
to the old release flow; the new pipeline owns publishing, so this is no
longer needed. |
| `.github/workflows/tagging.yml` | **Unchanged** | Dispatched by the
secure release pipeline. Not touched here. |
| `.github/workflows/push.yml` | **Unchanged** | PR + `merge_group` CI.
Preserved. |
| `.github/workflows/test.yml` | **Unchanged** | Reusable test workflow
called by `push.yml`. Preserved. |
| `.github/workflows/next-changelog.yml`, `integration-tests.yml`,
`external-message.yml` | **Unchanged** | Out of scope. |

## What's preserved

- PR and `merge_group` CI via `push.yml` (tests on Ubuntu + Windows
across Python 3.10/3.11/3.12, `fmt`, `check-manifest`).
- The reusable `test.yml` workflow.
- `tagging.yml` — still the dispatch target for the new secure release
pipeline.

## About the proposed rename

The original plan considered renaming a remaining release workflow to
`release-build-check.yml` if it retained dry-run / snapshot /
`merge_group` / PR-CI triggers. Neither of the two removed files carried
any such triggers (`release.yml` was tag-only; `release-test.yml` was
`workflow_dispatch`-only), so there was nothing to edit-and-rename.
Build verification for PRs already lives in `push.yml` / `test.yml`, so
a standalone `release-build-check.yml` would be a no-op and is
intentionally not created.

## Branch protection

If branch protection required status checks referenced the workflow
names **`Release`** or **`Release (TestPyPI)`**, those entries will need
to be removed from the branch protection settings when this merges —
they will no longer report.

## Test plan

- [ ] Confirm branch protection required status checks do not list
`Release` / `Release (TestPyPI)`; remove if present.
- [ ] Confirm the secure release pipeline in
`databricks/secure-public-registry-releases-eng` is the sole entry point
for cutting a new release.
- [ ] PR CI (`build` from `push.yml`) passes on this PR.

This pull request and its description were prepared by an AI agent on
behalf of @hectcastro.

Author: hectorcast-db