Remove some more "snapshot too old" vestiges.

Commit f691f5b8 removed the logic, but left behind some now-useless
Snapshot arguments to various AM-internal functions, and missed a couple
of comments.

Reported-by: Peter Geoghegan <pg@bowt.ie>
Discussion: https://postgr.es/m/CAH2-Wznj9qSNXZ1P1uWTUD_FeaTezbUazb416EPwi4Qr_jR_6A%40mail.gmail.com

src/backend/access/gin/ginbtree.c

@@ -78,7 +78,7 @@ ginTraverseLock(Buffer buffer, bool searchMode)
  */
 GinBtreeStack *
 ginFindLeafPage(GinBtree btree, bool searchMode,
-				bool rootConflictCheck, Snapshot snapshot)
+				bool rootConflictCheck)
 {
 	GinBtreeStack *stack;