Microsoft.ML package contains an empty Microsoft.ML dll

**Describe the bug**
Microsoft.ML.dll is shipped but contains no types.

When fixing https://github.com/dotnet/machinelearning/pull/6399 I noticed that we ship another package with an empty dll.

I tried to remove the dll from that package, but in doing so it would remove all the symbols.
https://github.com/ericstj/machinelearning/commit/d9eaee5c5f19e163bb0f6a972caac40a47833c80

This is due to the problem described in https://github.com/NuGet/Home/issues/10860#issuecomment-1287494174.

We can pursue a fix in NuGet then once we have it remove this useless assembly from the package.