Remove unnecessary feed (#79635)

Author: tmat

NuGet.config

@@ -9,8 +9,6 @@
     <add key="dotnet6" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet6/nuget/v3/index.json" />
     <!-- We need dotnet7-transport because we depend on Microsoft.CodeAnalysis.PerformanceSensitiveAnalyzers (>= 3.3.4-beta1.22504.1) -->
     <add key="dotnet7-transport" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet7-transport/nuget/v3/index.json" />
-    <!-- We need dotnet8 because we depend on Microsoft.ILVerification (>= 8.0.0-rtm.23523.3) -->
-    <add key="dotnet8" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet8/nuget/v3/index.json" />
     <!-- We need dotnet9 because we depend on Roslyn.Diagnostics.Analyzers (>= 3.11.0-beta1.24081.1)-->
     <add key="dotnet9" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet9/nuget/v3/index.json" />
     <add key="dotnet10" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet10/nuget/v3/index.json" />