Test:  remove hack for testing preview .NET SDK

Testing against a pre-release version of .NET 7 SDK brings in a dependency for Microsoft.NETCore.App.Host 3.1.31, which isn't released.  To work around this dependency, I implemented a [hack](https://github.com/NuGet/NuGet.Client/pull/4791/commits/02834c173867187a8044e4f5ed03b0e34b673b9f).  This hack should be removed once tests target the .NET 7 GA release.