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_formatter/src/other/parameters.rs

@@ -1,6 +1,5 @@
 use ruff_formatter::{format_args, write, FormatRuleWithOptions};
-use ruff_python_ast::Parameters;
-use ruff_python_ast::{AnyNodeRef, AstNode};
+use ruff_python_ast::{AnyNodeRef, Parameters};
 use ruff_python_trivia::{CommentLinePosition, SimpleToken, SimpleTokenKind, SimpleTokenizer};
 use ruff_text_size::{Ranged, TextRange, TextSize};
 
@@ -165,7 +164,7 @@ impl FormatNodeRule<Parameters> for FormatParameters {
                     token("*"),
                     vararg.format()
                 ]);
-                last_node = Some(vararg.as_any_node_ref());
+                last_node = Some(vararg.as_ref().into());
             } else if !kwonlyargs.is_empty() {
                 // Given very strange comment placement, comments here may not actually have been
                 // marked as `StarLeading`/`StarTrailing`, but that's fine since we still produce
@@ -201,7 +200,7 @@ impl FormatNodeRule<Parameters> for FormatParameters {
                     token("**"),
                     kwarg.format()
                 ]);
-                last_node = Some(kwarg.as_any_node_ref());
+                last_node = Some(kwarg.as_ref().into());
             }
 
             joiner.finish()?;