Run the extension index and security scan schedulers on a single cluster member

The check deciding whether the extension index is shared by all the cluster members now comes from XWIKI-25255, which adds isIndexShared() to the internal SolrConfiguration role (true when solr.type is remote). The extension indexing, cleanup and security scan are restricted to the cluster singleton node when remote observation is enabled and that method returns true, instead of testing solr.type here, so the changes relying on this assumption share one definition. The behaviour is unchanged.