Resolve TODO comment (#3233)

## Summary:
I added this TODO about a year ago. I don't know if the `Key` type it referred
to existed at that time, but it definitely doesn't exist now. I checked, and
the type of `extraKeys` really is `KeypadKey[]`, according to the parser.

Issue: none

## Test plan:

CI checks should pass.

Author: benchristel

Reviewers: handeyeco

Required Reviewers:

Approved By: handeyeco

Checks: ⏭️  1 check has been skipped, ✅ 10 checks were successful

Pull Request URL: #3233

Author: benchristel

.changeset/cyan-eels-buy.md

@@ -0,0 +1,5 @@
+---
+"@khanacademy/perseus-core": patch
+---
+
+Internal: Delete outdated TODO comment.

packages/perseus-core/src/data-schema.ts

@@ -501,7 +501,6 @@ export type PerseusExpressionWidgetOptions = {
     times: boolean;
     // What extra keys need to be displayed on the keypad so that the
     // question can be answerable without a keyboard (ie mobile)
-    // TODO: this is really Key[]
     extraKeys?: KeypadKey[];
     // visible label associated with the MathQuill field
     visibleLabel?: string;

packages/perseus-core/src/widgets/expression/derive-extra-keys.ts

@@ -1,10 +1,6 @@
 import * as KAS from "@khanacademy/kas";
 
-import {
-    type KeypadKey,
-    type KeypadConfiguration,
-    KeypadKeys,
-} from "../../keypad";
+import {type KeypadKey, KeypadKeys} from "../../keypad";
 
 import type {PerseusExpressionWidgetOptions} from "../../data-schema";
 
@@ -14,7 +10,7 @@ import type {PerseusExpressionWidgetOptions} from "../../data-schema";
  */
 function deriveExtraKeys(
     widgetOptions: PerseusExpressionWidgetOptions,
-): KeypadConfiguration["extraKeys"] {
+): KeypadKey[] {
     if (widgetOptions.extraKeys) {
         return widgetOptions.extraKeys;
     }