remove recommendation for deleted stylelint extension
Created by: sqs
According to https://github.com/stylelint/stylelint/pull/4438, the shinnn.stylelint extension was deleted because its creator abandoned it. The fork at https://marketplace.visualstudio.com/items?itemName=hex-ci.stylelint-plus is still maintained, but we can't use it because https://github.com/hex-ci/vscode-stylelint-plus/issues/8 means it would not support the newer version of the underlying stylelint tool and config we use.