Check PcdConfidentialComputingGuestAttr instead of calling
MemEncryptSevIsEnabled() and MemEncryptTdxIsEnabled() to figure
whenever SEV or TDX is enabled.
This allows to remove the MemEncryptSevLib + MemEncryptTdxLib
dependencies.
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>