Remove BindingKind::ComprehensionVar (#12347)

## Summary

This doesn't seem to be used anywhere. Maybe it mattered when we didn't
handle generator scopes properly?

crates/ruff_python_semantic/src/binding.rs

@@ -467,12 +467,6 @@ pub enum BindingKind<'a> {
     /// ```
     LoopVar,
 
-    /// A binding for a comprehension variable, like `x` in:
-    /// ```python
-    /// [x for x in range(10)]
-    /// ```
-    ComprehensionVar,
-
     /// A binding for a with statement variable, like `x` in:
     /// ```python
     /// with open('foo.py') as x: