fix: Remove unused import

Author: k-nasa

src/stream/stream/flatten.rs

@@ -1,8 +1,8 @@
 use pin_project_lite::pin_project;
 use std::pin::Pin;
 
-use crate::prelude::*;
-use crate::stream::stream::map::Map;
+
+
 use crate::stream::{IntoStream, Stream};
 use crate::task::{Context, Poll};