Remove MultiRenderer (#1955)

## Summary:
GTP is dead and MultiRenderer existed for GTP. Removing it simplifies things quite a bit.

Issue: [LEMS-2675](https://khanacademy.atlassian.net/browse/LEMS-2675)

## Test plan:
Shouldn't affect anything, no longer in use

[LEMS-2675]: https://khanacademy.atlassian.net/browse/LEMS-2675?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

Author: handeyeco

Reviewers: jeremywiebe

Required Reviewers:

Approved By: jeremywiebe

Checks: ✅ Publish npm snapshot (ubuntu-latest, 20.x), ✅ Check builds for changes in size (ubuntu-latest, 20.x), ✅ Check for .changeset entries for all changed files (ubuntu-latest, 20.x), ✅ Lint, Typecheck, Format, and Test (ubuntu-latest, 20.x), ✅ Cypress (ubuntu-latest, 20.x), ✅ Publish Storybook to Chromatic (ubuntu-latest, 20.x), ✅ .github/dependabot.yml

Pull Request URL: #1955

.changeset/cool-taxis-clap.md

@@ -0,0 +1,6 @@
+---
+"@khanacademy/perseus": major
+"@khanacademy/perseus-editor": major
+---
+
+Remove support for MultiRenderer

docs/architecture.md

@@ -55,17 +55,6 @@ with a short description and the main Perseus data type it accepts.
             article, but the scoring is not available outside of the renderer.
         </td>
     </tr>
-    <tr>
-        <td><code>MultiItemRenderer</code></td>
-        <td> <code>{ _multi: any }</code></td>
-        <td>
-            The `MultiItemRenderer` is a more advanced renderer. It accepts an
-            object representing a tree of `PerseusRenderer` objects. A `Shape`
-            object defines the structure of the tree and you must provide a
-            "callback" object of the same structure which defines how and where
-            to render each `PerseusRenderer` that appears in the tree.
-        </td>
-    </tr>
     <tr>
         <td><code>Renderer</code></td>
         <td><code>PerseusRenderer</code></td>