Remove duplicate XmlSerializerAppContextSwitchScope definition

src/libraries/System.Private.Xml/tests/XmlSerializer/XmlSerializerTests.cs

@@ -3091,56 +3091,3 @@ private static string GuessCachedName(string name)
         throw new ArgumentException($"Cannot guess cached field name from switch name '{name}'");
     }
 }
-
-internal sealed class XmlSerializerAppContextSwitchScope : IDisposable
-{
-    private readonly string _name;
-    private readonly string _cachedName;
-    private readonly bool _hadValue;
-    private readonly bool _originalValue;
-
-    public bool OriginalValue => _originalValue;
-    public bool CurrentValue => AppContext.TryGetSwitch(_name, out bool value) && value;
-
-    public XmlSerializerAppContextSwitchScope(string name, bool value, string cachedName = null)
-    {
-        _name = name;
-        _cachedName = cachedName ?? GuessCachedName(name);
-        _hadValue = AppContext.TryGetSwitch(name, out _originalValue);
-        AppContext.SetSwitch(name, value);
-        ClearCachedSwitch(_cachedName);
-    }
-
-    public void Dispose()
-    {
-        if (_hadValue)
-            AppContext.SetSwitch(_name, _originalValue);
-        else
-            // There's no "unset", so pick a default or false
-            AppContext.SetSwitch(_name, false);
-        ClearCachedSwitch(_cachedName);
-    }
-
-    private static void ClearCachedSwitch(string name)
-    {
-        Type t = Type.GetType("System.Xml.LocalAppContextSwitches, System.Private.Xml");
-        Assert.NotNull(t);
-
-        FieldInfo fi = t.GetField(name, BindingFlags.NonPublic | BindingFlags.Static);
-        Assert.NotNull(fi);
-        fi.SetValue(null, 0);
-    }
-
-    private static string GuessCachedName(string name)
-    {
-        // Switch names are typically of the form "Switch.System.Xml.SomeFeature"
-        // The cached field is typically "s_someFeature"
-        int idx = name.LastIndexOf('.');
-        if (idx > 0 && idx < name.Length - 1)
-        {
-            string feature = name.Substring(idx + 1);
-            return "s_" + char.ToLowerInvariant(feature[0]) + feature.Substring(1);
-        }
-        throw new ArgumentException($"Cannot guess cached field name from switch name '{name}'");
-    }
-}