amcheck: Remove unused GinScanItem->parentlsn field

amcheck: Remove unused GinScanItem->parentlsn field

The field was introduced by commit 14ffaece0fb5, but is unused and
unnecessary. So remove it.

Issues reported by Arseniy Mukhin, along with a proposed patch. Review
by Andrey M. Borodin, cleanup and minor improvements by me.

Author: Arseniy Mukhin <[email protected]>
Reviewed-by: Andrey M. Borodin <[email protected]>
Discussion: https://postgr.es/m/CAE7r3MJ611B9TE=YqBBncewp7-k64VWs+sjk7XF6fJUX77uFBA@mail.gmail.com