drm/xe: Remove unused FF_SLICE_CS_CHICKEN2

Commit e89f4967 ("drm/xe: Drop WA 16015675438") removed the only
user of that register and should have removed it. Remove it now.

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240306192128.1895603-1-lucas.demarchi@intel.com


Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>