StandaloneMmPkg/Core/Dispatcher: don't copy dispatched image twice

StandaloneMmPkg/Core/Dispatcher: don't copy dispatched image twice

The dispatcher uses the PE/COFF loader to load images into the heap,
but only does so after copying the entire image first, leading to
two copies being made for no good reason.

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