Review benchmark project

The [Rx.NET/Source/benchmarks/Benchmarks.System.Reactive/Benchmarks.System.Reactive.csproj](https://github.com/dotnet/reactive/blob/fb2b69dd232bfa1415bcf09ccfd5ea2f6d4fb992/Rx.NET/Source/benchmarks/Benchmarks.System.Reactive/Benchmarks.System.Reactive.csproj) project seems to have conditional sections pertaining to Rx 3.1.1 and 4.0. This suggests it's somewhat out of date.

Since taking over this project, I've not yet looked at this project, so I don't understand its role. It needs to be reviewed and, if necessary, updated.