delete needless use

src/prolog/arithmetic.rs

@@ -2,7 +2,6 @@ use prolog::ast::*;
 use prolog::fixtures::*;
 
 use std::cmp::{min, max};
-use std::rc::Rc;
 use std::vec::Vec;
 
 pub struct ArithExprIterator<'a> {

src/prolog/codegen.rs

@@ -9,7 +9,6 @@ use prolog::targets::*;
 use std::cell::Cell;
 use std::collections::HashMap;
 use std::mem::swap;
-use std::rc::Rc;
 use std::vec::Vec;
 
 pub struct CodeGenerator<'a, TermMarker> {