Delete Storybook stories that use randomly generated data (#1594)

These aren't particularly useful - the widgets they generate often look broken,
and it's hard to tell if there's a bug in the widget or the input data is just
something a human would never create.

I believe this completes the effort (started by @handeyeco) to move code out of
`perseus/src/widgets` and into specific widget subdirectories.

Issue: none

## Test plan:

`yarn test`

Author: benchristel

Reviewers: handeyeco, jeremywiebe

Required Reviewers:

Approved By: handeyeco

Checks: ❌ codecov/project, ✅ codecov/patch, ✅ Upload Coverage (ubuntu-latest, 20.x), ✅ Publish npm snapshot (ubuntu-latest, 20.x), ✅ Lint, Typecheck, Format, and Test (ubuntu-latest, 20.x), ✅ Cypress (ubuntu-latest, 20.x), ✅ Jest Coverage (ubuntu-latest, 20.x), ✅ Check for .changeset entries for all changed files (ubuntu-latest, 20.x), ✅ Check builds for changes in size (ubuntu-latest, 20.x), ✅ Publish Storybook to Chromatic (ubuntu-latest, 20.x), ✅ gerald

Pull Request URL: #1594

.changeset/proud-goats-provide.md

@@ -0,0 +1,7 @@
+---
+"@khanacademy/perseus-dev-ui": patch
+"@khanacademy/perseus": patch
+"@khanacademy/perseus-editor": patch
+---
+
+Internal: Remove Storybook stories that generate random widgets