Don't create MongoDB fulltext index when fulltext search is disabled

When using nuxeo.vcs.fulltext.search.disabled=true we want to have the fulltext index only in Elasticsearch so we don't need anymore to maintain the repository index named fulltext.

org.nuxeo.ecm.core.storage.mongodb.MongoDBRepository#initRepository must be updated to not create the index on startup when nuxeo.vcs.fulltext.search.disabled=true.

This way we can drop manually the index when switching fulltext search to Elasticsearch, so we don't waste resource to maintain this unused index.