Removed net standard 2.0

[ReeceHumphreys]

Removed the build folders related to net standard 2.0

• Followed the instructions from the csharp readme for macOS and everything else still built fine.
• Not sure if I did indeed remove everything properly as the csharp build system is confusing, so let me know if something doesn't make sense.

[pepone]

The main README need updates remove the corresponding section https://github.com/ReeceHumphreys/ice-demos/tree/removing-netstandard/csharp#building-and-running-the-demos-with-net-core-on-windows-linux-and-macos