[rust] Remove debug code (#6475)

* Remove debug code

This was added for testing in the recent genericize PR for flexbuffer Reader.

* Added alloc tests -> MapReader::{is_empty, index_key, len}

* Added , accessible through Deref to deprecation warning

rust/flexbuffers/src/reader/map.rs

@@ -71,8 +71,6 @@ impl<B: Buffer> std::fmt::Debug for MapReader<B> {
 impl<B: Buffer> MapReader<B> {
     /// Returns the number of key/value pairs are in the map.
     pub fn len(&self) -> usize {
-        let thing = self.clone();
-        println!("{:?}", &thing);
         self.length
     }