Revert "rbtree: add rb_destroy helper"

This reverts commit 7966b314b2f1f984a7cc4a2eab6598aa6ee023fd.
zookeeper: fix build failure in sheep/cluster/zookeeper.c

The commit 8b4a72f310f0e8a14d99187fc97066e1017759b3 "lib: rename
wrappers of pthread_rwlock" mistakenly inserted rb_destroy() in
zk_tree_destroy() function and broke build of zookeeper.

This fixes the issue.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>