Revert "add size_hint to mpsc receiver"

This reverts commit 0e4dc7d.

Author: taiki-e

futures-channel/src/mpsc/mod.rs

@@ -282,7 +282,7 @@ struct UnboundedInner<T> {
 
 #[derive(Debug)]
 struct BoundedInner<T> {
-    // Max buffer size of the channel.
+    // Max buffer size of the channel. If `None` then the channel is unbounded.
     buffer: usize,
 
     // Internal channel state. Consists of the number of messages stored in the
@@ -1125,10 +1125,6 @@ impl<T> Stream for Receiver<T> {
             }
         }
     }
-
-    fn size_hint(&self) -> (usize, Option<usize>) {
-        (0, self.inner.as_ref().map(|i| i.buffer))
-    }
 }
 
 impl<T> Drop for Receiver<T> {