Clean-up of DynamoModel.ShutDown methods

Hi @aparajit-pratap, this is a minor clean-up as part of [another larger change](https://github.com/DynamoDS/Dynamo/pull/2429). It removes unused ```EventArgs``` from ```DynamoModel.ShutDown``` and ```RevitDynamoModel.ShutDown``` methods (also some downstream methods).

Please have a look, thanks!