remove extra::rope

It's broken/unmaintained and needs to be rewritten to avoid managed
pointers and needless copies. A full rewrite is necessary and the API
will need to be redone so it's not worth keeping this around.

Closes #2236, #2744

src/libextra/extra.rs

@@ -74,7 +74,6 @@ pub mod deque;
 pub mod fun_treemap;
 pub mod list;
 pub mod priority_queue;
-pub mod rope;
 pub mod smallintmap;
 
 pub mod sort;