(core) Remove deprecated code

Summary:
The removed code is no longer reachable now that a new version of the plugin
API is being served by Grist on production.

Test Plan: Existing tests.

Reviewers: jarek

Reviewed By: jarek

Subscribers: jarek

Differential Revision: https://phab.getgrist.com/D4081

app/plugin/grist-plugin-api.ts

@@ -247,6 +247,8 @@ async function getMappingsIfChanged(data: any): Promise<WidgetColumnMap|null> {
  * Used by tests to wait for all pending requests to settle.
  *
  * TODO: currently only waits for requests for mappings.
+ *
+ * @internal
  */
 export async function testWaitForPendingRequests() {
   return await _activeRefreshReq;