Move experimental

[ericwindmill]

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

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