nir: call nir_metadata_preserve at nir_remove_unused_io_vars

nir: call nir_metadata_preserve at nir_remove_unused_io_vars

Without it we got a metadata assert:
deqp-vk: ../src/compiler/nir/nir_metadata.c:108: nir_metadata_check_validation_flag: Assertion `!(function->impl->valid_metadata & nir_metadata_not_properly_reset)' failed

if we try to use NIR_PASS(_, instead of NIR_PASS_V (that among other
things, do more validations).

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17609>