MdePkg: Follow PI1.4a to fix artificial limitation of SkuId range

MdePkg: Follow PI1.4a to fix artificial limitation of SkuId range

There is absolutely no reason to artificially limit the SKU range to 1-255.
PI1.4a spec fixed the artificial limitation.

This patch is to follow PI1.4a spec to remove PCD_MAX_SKU_ID definition,
the check to PCD_MAX_SKU_ID and the comments describes the limitation.

Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>