Remove assertion that circular buffer entries are monotonic

GetOldestXmin *can* go backwards, see its comment. For the record, spotted by
shardman's test_worker_failover_with_offline_neighbour test: apparently LR
launcher background worker gets low oldestxmin on its start because it is not
connected to any database.

Author: arssher

src/backend/access/transam/global_snapshot.c

@@ -282,7 +282,6 @@ GlobalSnapshotMapXmin(GlobalCSN snapshot_global_csn)
 
 	Assert(TransactionIdIsNormal(current_oldest_xmin));
 	Assert(TransactionIdIsNormal(previous_oldest_xmin));
-	Assert(TransactionIdFollowsOrEquals(current_oldest_xmin, previous_oldest_xmin));
 
 	gap = csn_seconds - last_csn_seconds;
 	offset = csn_seconds % gsXidMap->size;