Remove RefEquality (#6393)

## Summary

See discussion in
https://github.com/astral-sh/ruff/pull/6351#discussion_r1284996979. We
can remove `RefEquality` entirely and instead use a text offset for
statement keys, since no two statements can start at the same text
offset.

## Test Plan

`cargo test`

crates/ruff_python_ast/src/types.rs

@@ -1,5 +1,3 @@
-use std::ops::Deref;
-
 use crate::{Expr, Stmt};
 
 #[derive(Clone)]
@@ -7,78 +5,3 @@ pub enum Node<'a> {
     Stmt(&'a Stmt),
     Expr(&'a Expr),
 }
-
-#[derive(Debug)]
-pub struct RefEquality<'a, T>(pub &'a T);
-
-impl<'a, T> RefEquality<'a, T> {
-    // More specific implementation that keeps the `'a` lifetime.
-    // It's otherwise the same as [`AsRef::as_ref`]
-    #[allow(clippy::should_implement_trait)]
-    pub fn as_ref(&self) -> &'a T {
-        self.0
-    }
-}
-
-impl<'a, T> AsRef<T> for RefEquality<'a, T> {
-    fn as_ref(&self) -> &T {
-        self.0
-    }
-}
-
-impl<'a, T> Clone for RefEquality<'a, T> {
-    fn clone(&self) -> Self {
-        *self
-    }
-}
-
-impl<'a, T> Copy for RefEquality<'a, T> {}
-
-impl<'a, T> std::hash::Hash for RefEquality<'a, T> {
-    fn hash<H>(&self, state: &mut H)
-    where
-        H: std::hash::Hasher,
-    {
-        (self.0 as *const T).hash(state);
-    }
-}
-
-impl<'a, 'b, T> PartialEq<RefEquality<'b, T>> for RefEquality<'a, T> {
-    fn eq(&self, other: &RefEquality<'b, T>) -> bool {
-        std::ptr::eq(self.0, other.0)
-    }
-}
-
-impl<'a, T> Eq for RefEquality<'a, T> {}
-
-impl<'a, T> Deref for RefEquality<'a, T> {
-    type Target = T;
-
-    fn deref(&self) -> &T {
-        self.0
-    }
-}
-
-impl<'a> From<&RefEquality<'a, Stmt>> for &'a Stmt {
-    fn from(r: &RefEquality<'a, Stmt>) -> Self {
-        r.0
-    }
-}
-
-impl<'a> From<&RefEquality<'a, Expr>> for &'a Expr {
-    fn from(r: &RefEquality<'a, Expr>) -> Self {
-        r.0
-    }
-}
-
-impl<'a> From<RefEquality<'a, Stmt>> for &'a Stmt {
-    fn from(r: RefEquality<'a, Stmt>) -> Self {
-        r.0
-    }
-}
-
-impl<'a> From<RefEquality<'a, Expr>> for &'a Expr {
-    fn from(r: RefEquality<'a, Expr>) -> Self {
-        r.0
-    }
-}