SYSML21 — Constraint checkRequirementUsageSubrequirementSpecialization needs to exclude assumed requirements

The constraint checkRequirementUsageSubrequirementSpecialization requires that a composite RequirementUsage whose ownedType is a RequirementDefinition or RequirementUsage specialize RequirementCheck::subrequirements. However, RequirementCheck::subrequirements subsets RequirementCheck::constraints, which is the set of required constraints of the requirement. So it doesn't make sense for a RequirementUsage to specialize subrequirements if it is owned as an assumed constraint.