pgsql: Remove pre-order and post-order traversal logic for red-black tr

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: /messages/by-id/17735.1505003111@sss.pgh.pa.us

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/f80e782a6b4dcdea78f053f1505fff316f3a3289

Modified Files
--------------
src/backend/lib/rbtree.c | 129 +----------------------------------------------
src/include/lib/rbtree.h | 5 +-
2 files changed, 3 insertions(+), 131 deletions(-)

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