Don't remove converted files in local settings

When migrating local settings, we don't want to remove files that we
converted from as these settings will be created on demand and don't
represent the totality of user settings

Fixes https://gitlab.com/kicad/code/kicad/issues/9905

common/project/project_local_settings.cpp

@@ -44,6 +44,9 @@ PROJECT_LOCAL_SETTINGS::PROJECT_LOCAL_SETTINGS( PROJECT* aProject, const wxStrin
         m_SelectionFilter(),
         m_project( aProject )
 {
+    // Keep old files around
+    m_deleteLegacyAfterMigration = false;
+
     m_params.emplace_back( new PARAM_LAMBDA<std::string>( "board.visible_layers",
             [&]() -> std::string
             {