efi_loader: capsule: remove authentication data

If capsule authentication is disabled and yet a capsule file is signed,
its signature must be removed from image data to flush.
Otherwise, the firmware will be corrupted after update.

Fixes: 04be98bd ("efi: capsule: Add support for uefi capsule
	authentication")
Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>