This PR removes validation checks related to the definitions of item and part usages.

Previously, the implementation validated that an item usage is typed only by item definitions (or KerML structures). In particular, this prevented an item or part usage from being typed by an occurrence definition that is not an item definition.

While this restriction is mentioned in the specification in the Language Description subclauses for items and parts, there is actually no such restriction formally specified in the Metamodel. The property ItemUsage::itemDefinition subsets OccurrenceUsage::occurrenceDefinition, it does not redefine it, and ItemUsage has no validation constraints. So item usages should be able to be typed by occurrence definitions. (See also OMG issue SYSML21-418.)

In addition, the implementation validated that a part usage had exactly one part definition. However, this validation check is unnecessary, since a part usage always has an implied subsetting of Parts::parts, which provides typing by at least the base part definition Parts::Part.

The PR removes the validation checks for both item and part usages.