Remove scary message (flutter#1042)

... from the `README.md` that applied to 4-5 updates ago, only.

packages/cloud_firestore/README.md

@@ -8,8 +8,6 @@ For Flutter plugins for other Firebase products, see [FlutterFire.md](https://gi
 
 *Note*: This plugin is still under development, and some APIs might not be available yet. [Feedback](https://github.com/flutter/flutter/issues) and [Pull Requests](https://github.com/flutter/plugins/pulls) are most welcome!
 
-Recent versions (0.3.x and 0.4.x) of this plugin require [extensible codec functionality](https://github.com/flutter/flutter/pull/15414) that is not yet released to the [beta channel](https://github.com/flutter/flutter/wiki/Flutter-build-release-channels) of Flutter. If you're encountering issues using those versions, consider switching to the dev channel.
-
 ## Setup
 
 To use this plugin: