Remove dead & duplicated code (#11405)

* Remove dead code

* Make dead code happier

* DRY pickle additional data calculation

* Iterate

src/indexing/BaseEventIndexManager.ts

@@ -112,13 +112,6 @@ export default abstract class BaseEventIndexManager {
         throw new Error("Unimplemented");
     }
 
-    /**
-     * Check if our event index is empty.
-     */
-    public indexIsEmpty(): Promise<boolean> {
-        throw new Error("Unimplemented");
-    }
-
     /**
      * Check if the room with the given id is already indexed.
      *