[partition] Drop some re-parenting trickery

- Can't re-parent across threads easily
 - If device is made by immutableDeviceCopy(), then it's still owned by the
   PartitionCoreModule; giving it away to the widget is not a good idea.

src/modules/partition/gui/ChoicePage.cpp

@@ -882,11 +882,6 @@ ChoicePage::updateDeviceStatePreview()
     PartitionModel* model = new PartitionModel( m_beforePartitionBarsView );
     model->init( deviceBefore, m_core->osproberEntries() );
 
-    // The QObject parents tree is meaningful for memory management here,
-    // see qDeleteAll above.
-    deviceBefore->setParent( model );  // Can't reparent across threads
-    model->setParent( m_beforePartitionBarsView );
-
     m_beforePartitionBarsView->setModel( model );
     m_beforePartitionLabelsView->setModel( model );