Ninety-nine years to the day after it opened, the walls of the former General Motors assembly plant in Janesville started coming down.  Missouri-based Commercial Development Company is re-purposing the huge property.  It is expected to take 12-to-18 months to demolish the four-point-eight-million square foot building where thousands of people worked over the decades.  The assembly plant opened its doors May 1st, 1919, and was shut down nearly 10 years ago.  C-D-C bought the property in December with plans to redevelop it.  Owners say they have already received “significant interest” from prospective occupants.