SecurityPkg FmpAuthenticationLibRsa2048Sha256: Remove PCD reference

SecurityPkg FmpAuthenticationLibRsa2048Sha256: Remove PCD reference

PcdRsa2048Sha256PublicKeyBuffer is referenced but not used in the
library, that makes me a little confusing.
Actually, the PublicKeyData should be from the caller of
AuthenticateFmpImage() as input parameter, for example
EdkiiSystemCapsuleLib.

This patch is to remove the PCD reference in this library instance
to be aligned with FmpAuthenticationLibPkcs7 that does not reference
PcdPkcs7CertBuffer.

Cc: Chao Zhang <chao.b.zhang@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>