drm/i915/gt: Drop a few unused register definitions

Let's drop a few register definitions that are unused anywhere in the
driver today.  Since the referenced offsets are part of what is now
considered a multicast register region, the current definitions would
not be correct for use on any future platform.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Balasubramani Vivekanandan <balasubramani.vivekanandan@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221014230239.1023689-4-matthew.d.roper@intel.com