Eliminate Cirrus build status badge (flutter#140461)

We have no tasks running on Cirrus any more. This eliminates the "Build
Status - Cirrus" badge/link as well as the "Build status" link
reference, which was unused.

@CaseyHillers has filed a bug to add a Cocoon endpoint to replace it:
flutter#140460

## Pre-launch Checklist

- [X] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [X] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [X] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [X] I signed the [CLA].
- [X] I listed at least one issue that this PR fixes in the description
above.
- [X] I updated/added relevant documentation (doc comments with `///`).
- [ ] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [X] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#overview
[Tree Hygiene]: https://github.com/flutter/flutter/wiki/Tree-hygiene
[test-exempt]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#handling-breaking-changes
[Discord]: https://github.com/flutter/flutter/wiki/Chat

Author: cbracken

README.md

@@ -7,7 +7,6 @@
   </h1>
 </a>
 
-[![Build Status - Cirrus][]][Build status]
 [![Discord badge][]][Discord instructions]
 [![Twitter handle][]][Twitter badge]
 [![codecov](https://codecov.io/gh/flutter/flutter/branch/master/graph/badge.svg?token=11yDrJU2M2)](https://codecov.io/gh/flutter/flutter)
@@ -97,8 +96,6 @@ Information on how to get started can be found in our
 [contributor guide](CONTRIBUTING.md).
 
 [flutter.dev]: https://flutter.dev
-[Build Status - Cirrus]: https://api.cirrus-ci.com/github/flutter/flutter.svg
-[Build status]: https://cirrus-ci.com/github/flutter/flutter/master
 [Discord instructions]: https://github.com/flutter/flutter/wiki/Chat
 [Discord badge]: https://img.shields.io/discord/608014603317936148?logo=discord
 [Twitter handle]: https://img.shields.io/twitter/follow/flutterdev.svg?style=social&label=Follow