remove mixed=true attributes from element definitions to allow content models to omit mixed content models during specialization.

- learningAssesmentBody, learningContentBody, learningPlanBody, learningOverviewBody and learningSummaryBody elements are required

- prelreqs and closereqs element are missing child elements
- defaultSubject element should not have a collection-type attribute
- fix space in lomStructure value attribute contains a restricted value “-dita-use-conref-target”
- topicsubject element contains wrong definition for scope attribute
- lomStructure value attribute does not contain all possible values