Don't specify cache class in generated XML for vocabularies for TP 9.3+

Currently a cache defined by Studio contains a class="org.nuxeo.ecm.core.cache.InMemoryCacheImpl" specification:

    <cache name="vocab-WorkflowType-cache" class="org.nuxeo.ecm.core.cache.InMemoryCacheImpl">
      <option name="maxSize">1000</option>
      <option name="concurrencyLevel">500</option>
      <ttl>60</ttl>
    </cache>

After NXP-22786 this becomes unneeded as all caches will be in-memory and anyway what's specified is already the default.

So the class element should not be generated anymore (in VocabularyFeatureBuilder) for Nuxeo >= 9.3.

(I hope a future NXP fix will make all directory caches implicit and not needing explicit configuration.)