StandaloneMmPkg/Core: drop support for dispatching FVs into MM

StandaloneMmPkg/Core: drop support for dispatching FVs into MM

Remove the support that permits calls into the MM context to dispatch
firmware volumes that are not part of the initial standalone MM firmware
volume.

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>