Delete crossdac Dockerfile

[mthalman]

This crossdac Dockerfile is causing the Ubuntu pipeline to fail with the following error:

#5 9.814 error NU1202: Package dotnet-symbol 1.0.415602 is not compatible with netcoreapp3.1 (.NETCoreApp,Version=v3.1) / any. Package dotnet-symbol 1.0.415602 supports: net6.0 (.NETCoreApp,Version=v6.0) / any
#5 9.862 The tool package could not be restored.
#5 9.862 Tool 'dotnet-symbol' failed to install. This failure may have been caused by:
#5 9.862 
#5 9.862 * You are attempting to install a preview release and did not use the --version option to specify the version.
#5 9.862 * A package by this name was found, but it was not a .NET Core tool.
#5 9.862 * The required NuGet feed cannot be accessed, perhaps because of an Internet connection problem.
#5 9.862 * You mistyped the name of the tool.
#5 9.862 
#5 9.862 For more reasons, including package naming enforcement, visit https://aka.ms/failure-installing-tool

As you can see, it's using a 3.1 base image which is out of support. I don't see any usage of the associated image tag so I'm just deleting this very old Dockerfile.