Remove AstNode and AnyNode (#15479)

While looking into potential AST optimizations, I noticed the `AstNode`
trait and `AnyNode` type aren't used anywhere in Ruff or Red Knot. It
looks like they might be historical artifacts of previous ways of
consuming AST nodes?

- `AstNode::cast`, `AstNode::cast_ref`, and `AstNode::can_cast` are not
used anywhere.
- Since `cast_ref` isn't needed anymore, the `Ref` associated type isn't
either.

This is a pure refactoring, with no intended behavior changes.

crates/ruff_python_ast/src/lib.rs

@@ -4,7 +4,6 @@ use std::path::Path;
 pub use expression::*;
 pub use generated::*;
 pub use int::*;
-pub use node::AstNode;
 pub use nodes::*;
 
 pub mod comparable;