use tokio::try_join instead of futures::try_join

We don't use the `futures` crate much. Remove one of only two references
to it (tokio has the identical macro).

pageserver/src/walredo.rs

@@ -493,7 +493,7 @@ impl WalRedoProcess {
             Ok::<[u8; 8192], Error>(buf)
         };
 
-        let res = futures::try_join!(f_stdout, f_stdin)?;
+        let res = tokio::try_join!(f_stdout, f_stdin)?;
 
         let buf = res.0;