[csharp-dotnet2] REMOVED

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.4.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

Removes C# 2.0 support. See #116

Per Microsoft Product Lifecycle, support for .NET Framework 2.0 ended in 2011:

Support for .NET Framework 2.0 ended on July 12, 2011. .NET 3.5 SP1 is the only supported service pack level after this date. We strongly encourage customers to migrate to .NET Framework 3.5 SP1. For more information, please visit the .NET Framework Support Lifecycle Policy FAQ https://support.microsoft.com/help/17455

cc @OpenAPITools/openapi-generator-core-team @mandrean (2017/08), @jimschubert (2017/09)