Remove usage of deprecated std::error::Error methods (#1206) (#1245)

tokio-threadpool/src/park/default_park.rs

@@ -84,12 +84,8 @@ impl Unpark for DefaultUnpark {
 
 impl fmt::Display for ParkError {
     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
-        self.description().fmt(fmt)
+        write!(fmt, "unknown park error")
     }
 }
 
-impl Error for ParkError {
-    fn description(&self) -> &str {
-        "unknown park error"
-    }
-}
+impl Error for ParkError {}