Fix comments in DcTopologyModel and DcTopologyModelTest.

Signed-off-by: Landry Huet <landry.huet@supergrid-institute.com>

Author: SGI-Landry

iidm/iidm-impl/src/main/java/com/powsybl/iidm/network/impl/DcTopologyModel.java

@@ -29,7 +29,9 @@
  * It is only meant to be used in NetworkImpl.
  * DC buses are parts of the DC network with the same voltage, i.e. connected by
  * zero impedance links. The only links with zero impedance are closed DcSwitches
- * with non-zero resistance.
+ * with zero resistance.
+ * Only nodes with terminal create buses. In particular, disconnected nodes do
+ * not create buses.
  *
  */
 class DcTopologyModel implements MultiVariantObject {

iidm/iidm-impl/src/test/java/com/powsybl/iidm/network/impl/DcTopologyModelTest.java

@@ -1,7 +1,5 @@
 package com.powsybl.iidm.network.impl;
 
-//import com.powsybl.commons.PowsyblException;
-//import com.powsybl.commons.ref.RefChain;
 import com.powsybl.iidm.network.*;
 import org.junit.jupiter.api.Test;