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.
Does this need the ```EventArgs``` argument too? Looks like it does not, in which case the ```ShutdownRequestedEventHandler``` delegate signature can be changed as well.