Remove hard-coded "ok": true from REST responses

Fixes #4310.

I chose to remove only the `{"ok": true}` messages where the `true` was hard-coded, there are still 3 places where the `ok` field could be different: RestDeleteByQueryAction, RestExplainAction, and RestClearScrollAction. In these the actual success of the action is returned.

I'm happy to remove those if we decide we don't want them.