Microsoft.ML package contains an empty Microsoft.ML dll

[ericstj]

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

When fixing #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.
ericstj@d9eaee5

This is due to the problem described in NuGet/Home#10860 (comment).

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