drm/i915: Drop duplicate WaDisable4x2SubspanOptimization:hsw

Same workaround was listed two times - once under the Gen7 block and once
under the Haswell section.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210507084926.2423003-1-tvrtko.ursulin@linux.intel.com