[3.13] Clean up redundant ifdef in list getitem (GH-128257) (#128276)

Clean up redundant ifdef in list getitem (GH-128257)

It's already inside a `Py_GIL_DISABLED` block so the `#else` clause is always unused.
(cherry picked from commit 42f7a00ae8b6b3fa09115e24b9512216c6c8978e)

Co-authored-by: da-woods <dw-git@d-woods.co.uk>