StandaloneMmPkg: drop unused PCD PcdStandaloneMmEnable

StandaloneMmPkg: drop unused PCD PcdStandaloneMmEnable

The PCD PcdStandaloneMmEnable is unused, and shouldn't exist in the
first place since the value is implied by the context (it is never
valid to set it to FALSE for standalone MM or TRUE for traditional
MM). So drop it.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: "Yao, Jiewen" <jiewen.yao@intel.com>
Reviewed-by: Achin Gupta <achin.gupta@arm.com>