Remove outdated comment

Fixes #96315

VS Code now calls `.delete` on the backup for you

src/vs/vscode.proposed.d.ts

@@ -1459,9 +1459,6 @@ declare module 'vscode' {
 		 * are displaying the document in the same state is saved in. This usually means reloading the file from the
 		 * workspace.
 		 *
-		 * During `revert`, your extension should also clear any backups for the custom editor. Backups are only needed
-		 * when there is a difference between an editor's state in VS Code and its save state on disk.
-		 *
 		 * @param document Document to revert.
 		 * @param cancellation Token that signals the revert is no longer required.
 		 *