Remove unused implementation of alpha_eq

lang/syntax/src/common/de_bruijn.rs

@@ -2,8 +2,6 @@ use std::fmt;
 use std::ops::{Bound, RangeBounds};
 use std::rc::Rc;
 
-use super::equiv::*;
-
 /// Two-dimensional De-Bruijn index
 ///
 /// The first component counts the number of binder lists in scope between the variable
@@ -155,12 +153,6 @@ impl<R: ShiftRange> ShiftRangeExt for R {
     }
 }
 
-impl AlphaEq for Idx {
-    fn alpha_eq(&self, other: &Self) -> bool {
-        self.fst == other.fst && self.snd == other.snd
-    }
-}
-
 impl fmt::Display for Idx {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         write!(f, "{}.{}", self.fst, self.snd)