Remove pruned packages from netstandard2.1 which were listed in packageoverrides but not present in references

Author: ericstj

src/Tasks/Microsoft.NET.Build.Tasks/FrameworkPackages/FrameworkPackages.netstandard2.1.cs

@@ -15,7 +15,6 @@ internal static class NETStandard21
         {
             { "System.Buffers", "4.6.1" },
             { "System.Collections.Concurrent", "4.3.0" },
-            { "System.Collections.Immutable", "1.4.0" },
             { "System.ComponentModel", "4.3.0" },
             { "System.ComponentModel.Composition", "4.5.0" },
             { "System.ComponentModel.EventBasedAsync", "4.3.0" },
@@ -27,20 +26,13 @@ internal static class NETStandard21
             { "System.Net.WebHeaderCollection", "4.3.0" },
             { "System.Numerics.Vectors", "4.6.1" },
             { "System.ObjectModel", "4.3.0" },
-            { "System.Private.DataContractSerialization", "4.3.0" },
             { "System.Reflection.DispatchProxy", "4.5.1" },
             { "System.Reflection.Emit", "4.7.0" },
             { "System.Reflection.Emit.ILGeneration", "4.7.0" },
             { "System.Reflection.Emit.Lightweight", "4.7.0" },
-            { "System.Reflection.TypeExtensions", "4.3.0" },
-            { "System.Runtime.Loader", "4.3.0" },
             { "System.Runtime.Numerics", "4.3.0" },
             { "System.Runtime.Serialization.Json", "4.3.0" },
-            { "System.Security.AccessControl", "4.4.0" },
-            // this package was listed in the package overrides.txt for netstandard2.1, but it is not actually in the targeting pack
-            // { "System.Security.Cryptography.Xml", "4.4.0" },
             { "System.Security.Principal", "4.3.0" },
-            { "System.Security.Principal.Windows", "4.4.0" },
             { "System.Threading", "4.3.0" },
             { "System.Threading.Tasks.Extensions", "4.6.3" },
             { "System.Threading.Tasks.Parallel", "4.3.0" },