Remove old packages

[Tratcher]

These packages have been replaced. We left the existing ones until we could get tooling support and covert projects over. Remove these packages before RC2 ships, after https://github.com/aspnet/Coherence-Signed/issues/222.
Microsoft.AspNetCore.IISPlatformHandler
dotnet-publish-iis
And associated tests.
@Eilon @muratg

[muratg]

I guess this consists of removing these from the build script for now? GitHub and CI changes can probably come in later.

Don't forget to update packages.csv :)

[Tratcher]

I'd rather remove the sources before RC2 ships to avoid confusion of what shipped. We should be ready this week.