intel/compiler: Drop variable group size lowering

This backend lowering code has been dead since the removal of i965 -
nothing in the current source tree ever sets the flag.

This is handled by iris_setup_uniforms() and crocus_setup_uniforms().
Variable group size does not appear to be a feature in anv.

Reviewed-by: Marcin Ślusarz <marcin.slusarz@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18055>

src/intel/compiler/brw_fs.h

@@ -372,7 +372,6 @@ public:
    int *push_constant_loc;
 
    fs_reg subgroup_id;
-   fs_reg group_size[3];
    fs_reg scratch_base;
    fs_reg frag_depth;
    fs_reg frag_stencil;