StandaloneMmPkg/StandaloneMmCoreEntryPoint: remove bogus ASSERT_EFI_ERROR()s

StandaloneMmPkg/StandaloneMmCoreEntryPoint: remove bogus ASSERT_EFI_ERROR()s

ASSERT_EFI_ERROR (x) is a shorthand for ASSERT(!EFI_ERROR(x)), and so
it should only be used with EFI_STATUS type expressions.

So drop two instances that operate on other types, since neither looks
particularly useful.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Supreeth Venkatesh <supreeth.venkatesh@arm.com>
Reviewed-by: Achin Gupta <achin.gupta@arm.com>