drm/i915/gt: Remove unused function prototype

Remove unused function prototype; intel_gt_create_kobj()

Cc: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221003170242.1246830-1-gwan-gyeong.mun@intel.com