gui/artwork: Don't destroy objects twice

The file filter and preview widget are both cleaned up when the file
chooser dialog is destroyed.

Signed-off-by: Anna Schumaker <Anna@NoWheyCreamery.com>

gui/artwork.c

@@ -148,6 +148,4 @@ void __artwork_select_cover(GtkButton *button)
 	}
 
 	gtk_widget_destroy(dialog);
-	gtk_widget_destroy(preview);
-	g_object_unref(filter);
 }