Removes the remaining samples in `experimental`. They're now in the
flutter/demos repository.
*List which issues are fixed by this PR. For larger changes, raising an
issue first helps
reduce redundant work.*
#2520
## 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 `///`).
