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_compiler.h

@@ -105,12 +105,6 @@ struct brw_compiler {
     */
    bool supports_shader_constants;
 
-   /**
-    * Whether or not the driver wants variable group size to be lowered by the
-    * back-end compiler.
-    */
-   bool lower_variable_group_size;
-
    /**
     * Whether indirect UBO loads should use the sampler or go through the
     * data/constant cache.  For the sampler, UBO surface states have to be set