Retire VeggieSeasons (really this time) (#2724)

Removes VeggieSeaons 

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [x] I have added sample code updates to the [changelog].
- [x] I updated/added relevant documentation (doc comments with `///`).

---------

Co-authored-by: Loïc Sharma <737941+loic-sharma@users.noreply.github.com>

experimental/README.md

@@ -3,34 +3,3 @@
 The `experimental` directory has been deprecated. All samples within have been deleted or moved to [github.com/flutter/demos][]
 
 
----
-
-
-# Master channel samples
-
-In this directory, you'll find samples that target Flutter's master channel.
-They may take advantage of new SDK features that haven't landed in the
-stable channel, and they may crash, lock up your machine, or otherwise fail to
-run correctly. In other words, consider everything in this directory to be an
-experiment.
-
-We're maintaining these samples here so that folks can see some of Flutter's
-upcoming features as they evolve and get a sense for how they'll eventually
-be used. If you'd like to run the apps, make sure to switch to the master
-channel first:
-
-```bash
-flutter channel master
-flutter upgrade
-```
-
-When you're done, use this command to return to the safety of the stable
-channel:
-
-```bash
-flutter channel stable
-```
-
-## Want more info about the repo in general?
-
-See the [README](../README.md) in the repo's root folder.