Revert "Added the main of a composite as a child element (it coult be a spec, an implem or an instance, with constraints and preferences), THIS IS NOT PARSED in ApAM 0.0.7, the old main attribute is still used until ApAM 0.0.8"

This reverts commit c382f8528fcee4a7a582e76363c49e7d9dedb58a.