include/cpp-btree: drop node handle support

include/cpp-btree: drop node handle support

this is a leftover in 73f2d70fc8bdbb4a963483ded0487dd744def215,

the node support is useful if we want to move a node around in a
node-based container. but in the case of b-tree, multiple elements
are packed into a single node/block, so we cannot benefit from this
feature, i think. so just drop all of them.

Signed-off-by: Kefu Chai <kchai@redhat.com>