aco/isel: Remove some dead code

exported_pos was always initialized to true (due to the is_pos argument
of the first export_vs_varying call being true), so none of this code has
any effect.

Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7102>