Revert "docs: add undeclaration on drop documentation for Rust"

This reverts commit c9ff21c.

Author: wyfo

content/docs/migration_1.0/Rust.md

@@ -107,36 +107,6 @@ session.close().await.unwrap();
 subscriber_task.await.unwrap()
 ```
 
-## Callbacks run in background until session is closed
-
-Session entities, e.g. subscribers, declared with callbacks are no more undeclared when they are dropped; there is no more need to keep a variable around when the intent is to have it run until the session is closed.
-
-```rust
-let session = zenoh::open(zenoh::config::peer()).await. unwrap();
-session
-    .declare_subscriber("key/ expression")
-    .callback(|sample| { println!("Received: {} {:?}", sample. key_expr(), sample. payload()) })
-    .await
-    .unwrap();
-// subscriber run in background until the session is closed
-// no need to keep a variable around
-```
-
-If you still want the entity to be undeclared when dropped, you can simply use `with` instead of `callback`; it may just require you to annotate the callback, as type inference is not as good as with `callback` method.
-
-```rust
-let session = zenoh::open(zenoh::config::peer()).await. unwrap();
-let subscriber = session
-    .declare_subscriber("key/ expression")
-    // annotation needed
-    .with(|sample: Sample| { println!("Received: {} {:?}", sample. key_expr(), sample. payload()) })
-    .await
-    .unwrap();
-// subscriber is undeclared when dropped
-```
-
-*Going into details, a new method `undeclare_on_drop(bool)` – default to `true`, has been added to the builders, and `callback(cb)` is now simply a shortcut to `with(cb).undeclare_on_drop(false)`. You should rarely need to call this method directly however.*
-
 ## Value is gone, long live ZBytes
 
 We have replaced `Value` with `ZBytes` and `Encoding` , and added a number of conversion implementations such that user structs can be serialized into `ZBytes`, sent via Zenoh, and de-serialized from `ZBytes` with ease.