Remove customizable reference enum names (#15647)

The AST generator creates a reference enum for each syntax group — an
enum where each variant contains a reference to the relevant syntax
node. Previously you could customize the name of the reference enum for
a group — primarily because there was an existing `ExpressionRef` type
that wouldn't have lined up with the auto-derived name `ExprRef`. This
follow-up PR is a simple search/replace to switch over to the
auto-derived name, so that we can remove this customization point.

crates/ruff_python_semantic/src/nodes.rs

@@ -79,7 +79,7 @@ impl<'a> Index<NodeId> for Nodes<'a> {
 ///
 /// TODO(charlie): Replace with [`ruff_python_ast::AnyNodeRef`]. This requires migrating
 /// the rest of the codebase to use [`ruff_python_ast::AnyNodeRef`] and related abstractions,
-/// like [`ruff_python_ast::ExpressionRef`] instead of [`Expr`].
+/// like [`ruff_python_ast::ExprRef`] instead of [`Expr`].
 #[derive(Copy, Clone, Debug, PartialEq)]
 pub enum NodeRef<'a> {
     Stmt(&'a Stmt),