[ty] Remove GenericAlias-related todo type (#21728)

## Summary

If you manage to create an `typing.GenericAlias` instance without us
knowing how that was created, then we don't know what to do with this in
a type annotation. So it's better to be explicit and show an error
instead of failing silently with a `@Todo` type.

## Test Plan

* New Markdown tests
* Zero ecosystem impact

crates/ty_python_semantic/src/types.rs

@@ -7372,9 +7372,6 @@ impl<'db> Type<'db> {
                 Some(KnownClass::TypeVarTuple) => Ok(todo_type!(
                     "Support for `typing.TypeVarTuple` instances in type expressions"
                 )),
-                Some(KnownClass::GenericAlias) => Ok(todo_type!(
-                    "Support for `typing.GenericAlias` instances in type expressions"
-                )),
                 _ => Err(InvalidTypeExpressionError {
                     invalid_expressions: smallvec::smallvec_inline![
                         InvalidTypeExpression::InvalidType(*self, scope_id)