drm/i915/kbl: Drop pre-production revision from stepping table

We're long past the point where we need to care about pre-production
hardware, and we already warn the user and taint the kernel if we detect
the driver is being loaded on pre-production hardware.

Bspec: 18329
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210713193635.3390052-5-matthew.d.roper@intel.com