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!

P/S: As this is related to Revit, I will also merge these changes into ```Revit2015``` when you're done.
Unfortunately that is required by another ```RequestAlternateContextClear```, which I cannot changed (due to it being defined as a ```EventHandler```). That's why I moved the common bits out into the parameterless ```CleanupVisualizations``` below.