[flutter_markdown] Remove build status from README.md

[nilsreichardt]

The build status badge for flutter_markdown is not working anymore and redirects to a 404 page. I assume the build status badge isn't used anymore and can be removed.

Closes flutter/flutter#100609

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
• 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.
• All existing and new tests are passing.

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[nilsreichardt]

@stuartmorgan This PR should fix the missing points of #1209.

[stuartmorgan]

LGTM, thanks!

[stuartmorgan]

@domesticmouse for secondary review.