Knip 4: remove MovablePoint duplicate export (#1774)

## Summary:
`MovablePoint` was being exported twice: `export class MovablePoint` and `export default MovablePoint`

Found by Knip.

Author: handeyeco

Reviewers: benchristel, jeremywiebe

Required Reviewers:

Approved By: jeremywiebe

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

Pull Request URL: #1774

.changeset/short-sheep-happen.md

@@ -0,0 +1,5 @@
+---
+"@khanacademy/perseus": patch
+---
+
+Remove duplicate export on MovablePoint

packages/perseus/src/interactive2/movable-point.tsx

@@ -801,5 +801,3 @@ function getKey(eventName: string, id: string): string {
 function getEventName(key: string): string {
     return key.split(":")[0];
 }
-
-export default MovablePoint;