Small cleanups in System.Console.cs file

Cleanup:
Removed unused using statements. 
Used var instead of Stream.

Local build succesful.
In general we only use var when the object is being constructed (i.e. var stream = new Stream()). We don't want to make assumptions based on the variable name. For example inputStream could be of a type called InputStream or MemoryStream or any other stream and we want to be explicit about this and not just assume based on the variable name. We read and maintain this code far more then we write it so we want to be more explicit to increase readability.