Qt: avoid a crash with plugins dialog.

(cherry picked from commit 3f5c8318)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/gui/qt4/dialogs/plugins.cpp

@@ -42,8 +42,6 @@ PluginDialog *PluginDialog::instance = NULL;
 
 PluginDialog::PluginDialog( intf_thread_t *_p_intf ) : QVLCFrame( _p_intf )
 {
-    setAttribute( Qt::WA_DeleteOnClose );
-
     setWindowTitle( qtr( "Plugins and extensions" ) );
     QGridLayout *layout = new QGridLayout( this );