Remove pre-order and post-order traversal logic for red-black trees.

This code isn't used, and there's no clear reason why anybody would ever
want to use it.  These traversal mechanisms don't yield a visitation order
that is semantically meaningful for any external purpose, nor are they
any faster or simpler than the left-to-right or right-to-left traversals.
(In fact, some rough testing suggests they are slower :-(.)  Moreover,
these mechanisms are impossible to test in any arm's-length fashion; doing
so requires knowledge of the red-black tree's internal implementation.
Hence, let's just jettison them.

Discussion: https://postgr.es/m/17735.1505003111@sss.pgh.pa.us

src/include/lib/rbtree.h

@@ -35,9 +35,7 @@ typedef struct RBTree RBTree;
 typedef enum RBOrderControl
 {
 	LeftRightWalk,				/* inorder: left child, node, right child */
-	RightLeftWalk,				/* reverse inorder: right, node, left */
-	DirectWalk,					/* preorder: node, left child, right child */
-	InvertedWalk				/* postorder: left child, right child, node */
+	RightLeftWalk				/* reverse inorder: right, node, left */
 } RBOrderControl;
 
 /*
@@ -52,7 +50,6 @@ struct RBTreeIterator
 	RBTree	   *rb;
 	RBNode	   *(*iterate) (RBTreeIterator *iter);
 	RBNode	   *last_visited;
-	char		next_step;
 	bool		is_over;
 };