intel/compiler: Remove cs_prog_data->threads

At this point all drivers are doing this math on their own -- since
most of them need to cover the variable group size case, in which at
compile time the group size (and number of threads) is not defined.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4504>