glsl: drop the dce of global vars from GLSL IR linker

glsl: drop the dce of global vars from GLSL IR linker

All this does is compilcate things such as forcing us to set
var->data.always_active_io in the glsl linker. Just let NIR clean
these up for us instead.

A Zink test hits a new assert but this is not a regression it just
uncovers an existing mesa bug.

Reviewed-by: Emma Anholt <emma@anholt.net>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22846>