UefiCpuPkg/MpInitLib: use PcdConfidentialComputingAttr to check SEV status

UefiCpuPkg/MpInitLib: use PcdConfidentialComputingAttr to check SEV status

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=3275

Previous commit introduced a generic confidential computing PCD that can
determine whether AMD SEV-ES is enabled. Update the MpInitLib to drop the
PcdSevEsIsEnabled in favor of PcdConfidentialComputingAttr.

Cc: Michael Roth <michael.roth@amd.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Rahul Kumar <rahul1.kumar@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: James Bottomley <jejb@linux.ibm.com>
Cc: Min Xu <min.m.xu@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Erdem Aktas <erdemaktas@google.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Ray Ni <ray.ni@intel.com>
Suggested-by: Jiewen Yao <jiewen.yao@intel.com>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>