Remove unnecessary "head" arguments from some dlist/slist functions.

dlist_delete, dlist_insert_after, dlist_insert_before, slist_insert_after
do not need access to the list header, and indeed insisting on that negates
one of the main advantages of a doubly-linked list.

In consequence, revert addition of "cache_bucket" field to CatCTup.

src/include/utils/catcache.h

@@ -82,7 +82,6 @@ typedef struct catctup
 	 * lookups.
 	 */
 	dlist_node	cache_elem;		/* list member of per-bucket list */
-	dlist_head *cache_bucket;	/* containing bucket dlist */
 
 	/*
 	 * The tuple may also be a member of at most one CatCList.	(If a single