remove unused thing

src/exit.rs

@@ -1,6 +1,5 @@
 use crate::Position;
 use std::str::FromStr;
-use std::error::Error;
 use std::fmt;
 use std::fmt::{Display, Formatter};
 
@@ -58,8 +57,6 @@ pub struct Exit {
     pub effect: Transition,
 }
 
-impl Error for Exit {}
-
 impl FromStr for Exit {
     type Err = String;