Revert change to export utils getObjKey delimiter

It turns out this is exposed to external consumers and caused some
integration tests to fail. The unrelated saved objects service getObjKey
function is internal only and uses a different delimiter.

src/core/server/saved_objects/export/utils.ts

@@ -10,7 +10,7 @@ import { SavedObject } from '../../../types';
 
 export type SavedObjectComparator = (a: SavedObject, b: SavedObject) => number;
 
-export const getObjKey = (obj: SavedObject) => `${obj.type}:${obj.id}`;
+export const getObjKey = (obj: SavedObject) => `${obj.type}|${obj.id}`;
 
 export const byIdAscComparator: SavedObjectComparator = (a: SavedObject, b: SavedObject) =>
   a.id > b.id ? 1 : -1;