drm/i915: Remove pre-production pooled-EU w/a for Broxton

WaEnablePooledEuFor2x6 only applies to preproduction models, unsupported
since commit 0102ba1f

 ("drm/i915: Add early BXT sdv to the list of
preproduction machines").

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171114135116.30036-1-chris@chris-wilson.co.uk
Reviewed-by: David Weinehall <david.weinehall@linux.intel.com>