Remove btree page items after page unlink

 Remove btree page items after page unlink
 
 Currently, page unlink leaves remaining items "as is", but replay of
 corresponding WAL-record re-initializes page leaving it with no items.
 For the sake of consistency, this commit makes primary delete all the items
 during page unlink as well.
 
 Thanks to this change, we now don't mask contents of deleted btree page for
 WAL consistency checking.
 
 Discussion: https://postgr.es/m/CAPpHfdt_OTyQpXaPJcWzV2N-LNeNJseNB-K_A66qG%3DL518VTFw%40mail.gmail.com
 Author: Alexander Korotkov
 Reviewed-by: Peter Geoghegan