cbtree: remove broken and unused cb_unlink

cbtree: remove broken and unused cb_unlink

cb_unlink is broken once a node is no longer self-referential
due to subsequent insertions.  This is a consequence of an
intrusive implementation and I'm not sure if it's easily fixable
while retaining our cache-friendly intrusive property (I've
tried for several hours in another project).

In any case, we're not using cb_unlink anywhere in our codebase,
just get rid of it to avoid misleading future readers.

Signed-off-by: Eric Wong <e@80x24.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>