Removed unnecessary "/admin" suffix from remote Solr instance admin URL

For quite quite a long time now, the Solr /admin URL suffix indeed
redirects to the Solr base context (see
https://issues.apache.org/jira/browse/SOLR-3337)

source/net/yacy/cora/federate/solr/instance/RemoteInstance.java

@@ -284,6 +284,9 @@ public class RemoteInstance implements SolrInstance {
         return o instanceof RemoteInstance && ((RemoteInstance) o).solrurl.equals(this.solrurl);
     }
 
+    /**
+     * @return the administration URL of the remote Solr instance
+     */
     public String getAdminInterface() {
         final InetAddress localhostExternAddress = Domains.myPublicLocalIP();
         final String localhostExtern = localhostExternAddress == null ? "127.0.0.1" : localhostExternAddress.getHostAddress();
@@ -292,7 +295,7 @@ public class RemoteInstance implements SolrInstance {
         if (p < 0) p = u.indexOf("127.0.0.1",0);
         if (p < 0) p = u.indexOf("0:0:0:0:0:0:0:1",0);
         if (p >= 0) u = u.substring(0, p) + localhostExtern + u.substring(p + 9);
-        return u + (u.endsWith("/") ? "admin/" : "/admin/");
+        return u;
     }
 
     @Override