no blanket prohibition in civil cases against a party's calling a witness who the party knows will assert his Fifth Amendment privilege, see, e.g., Brinks, Inc. v. City of New York, 717 F.2d 700, 708-10 (2d Cir. 1983); see also LiButti v. United States, 107 F.3d 110 (2d Cir. 1997), neither is this Court req