Remove the "snapshot too old" feature.

Remove the old_snapshot_threshold setting and mechanism for producing
the error "snapshot too old", originally added by commit 848ef42b.
Unfortunately it had a number of known problems in terms of correctness
and performance, mostly reported by Andres in the course of his work on
snapshot scalability.  We agreed to remove it, after a long period
without an active plan to fix it.

This is certainly a desirable feature, and someone might propose a new
or improved implementation in the future.

Reported-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CACG%3DezYV%2BEvO135fLRdVn-ZusfVsTY6cH1OZqWtezuEYH6ciQA%40mail.gmail.com
Discussion: https://postgr.es/m/20200401064008.qob7bfnnbu4w5cw4%40alap3.anarazel.de
Discussion: https://postgr.es/m/CA%2BTgmoY%3Daqf0zjTD%2B3dUWYkgMiNDegDLFjo%2B6ze%3DWtpik%2B3XqA%40mail.gmail.com

src/backend/access/gist/gistget.c

@@ -346,7 +346,6 @@ gistScanPage(IndexScanDesc scan, GISTSearchItem *pageItem,
 	PredicateLockPage(r, BufferGetBlockNumber(buffer), scan->xs_snapshot);
 	gistcheckpage(scan->indexRelation, buffer);
 	page = BufferGetPage(buffer);
-	TestForOldSnapshot(scan->xs_snapshot, r, page);
 	opaque = GistPageGetOpaque(page);
 
 	/*