Do not allow to subclassify an `EnumerationDefinition` by another `EnumerationDefinition`

[jerome-obeo]

As it is mentioned in the first paragraph of section 7.8.2 Enumeration Definitions and Usages of the SysMLv2 specification document:

However, an enumeration definition must not subclassify another enumeration definition.

We need to remove the New Subclassification edge tool in this situation: