update aspnetcore nuget feeds (#2285)

frameworks/CSharp/aspnetcore/Benchmarks/NuGet.Config

@@ -1,8 +1,7 @@
-<?xml version="1.0" encoding="utf-8"?>
+<?xml version="1.0" encoding="utf-8"?>
 <configuration>
   <packageSources>
     <clear />
-    <add key="AspNetVNext" value="https://dotnet.myget.org/F/aspnet1/api/v3/index.json" />
     <add key="NuGet" value="https://api.nuget.org/v3/index.json" />
   </packageSources>
 </configuration>