Remove equivalent set of values on ProductContent and and ProductPresentation #123

Implementors can define additional instances of the coos:ProductContent of coos:ProductPresentation for their specific needs.

COOS define a formal equivalence on ProductContent and ProductPresentation to the closed list of possible values.
Consequently, implementors cannot exte,d the list with their own values:

This formal equivalence should be deleted and replaced by skos:inScheme value productContents