Cleanup more usings.

src/Avalonia.Base/Data/Core/Plugins/AvaloniaPropertyAccessorPlugin.cs

@@ -2,7 +2,6 @@
 // Licensed under the MIT license. See licence.md file in the project root for full license information.
 
 using System;
-using System.Reactive.Linq;
 using System.Runtime.ExceptionServices;
 
 namespace Avalonia.Data.Core.Plugins

src/Avalonia.Base/Data/Core/PropertyAccessorNode.cs

@@ -2,8 +2,6 @@
 // Licensed under the MIT license. See licence.md file in the project root for full license information.
 
 using System;
-using System.Linq;
-using System.Reactive.Linq;
 using Avalonia.Data.Core.Plugins;
 
 namespace Avalonia.Data.Core