Remove "core" repos from being registered in MCR

Now that .NET Core 3.1 is EOL and container images are now longer maintained, the syndication to the old ["core" repos](https://github.com/dotnet/dotnet-docker/issues/2375) is no longer needed. .NET Core 3.1 was the last remaining version that was still be syndicated to those repos.

The syndication of images is removed by https://github.com/dotnet/dotnet-docker/pull/4275 but there is remaining work to remove the registration of those repos from MCR.

This means the following directories can be deleted:
* https://github.com/microsoft/mcr/tree/main/teams/dotnet/core
* https://github.com/microsoft/mcr/tree/main/teams/dotnet/core-nightly