[cleanup] remove stale presubmit template (#8979)

The template's been stale for a bit. Removing to avoid future confusion.

---

Review the contribution guidelines below:

<details>
  <summary>Contribution guidelines:</summary><br>

- See
  our [contributor guide](../CONTRIBUTING.md) and
  the [Flutter organization contributor guide]([https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md)
  for general expectations for PRs.
- Larger or significant changes should be discussed in an issue before creating a PR.
- Dart contributions to our repos should follow the [Dart style guide](https://dart.dev/guides/language/effective-dart) and use
  `dart format`.
- Java and Kotlin contributions should strive to follow Java and Kotlin best
  practices ([discussion](#8098)).

</details>

Author: pq

.github/workflows/presubmit.yaml

@@ -1,5 +1,3 @@
-# Do not edit; instead, modify presubmit.yaml.template, and run './bin/plugin generate'.
-
 # Copyright 2020 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.