Remove unused Constructor

The constructor was needed to override IsReadonly. This is not needed anymore.

src/Avalonia.Controls.DataGrid/DataGridTemplateColumn.cs

@@ -1,4 +1,4 @@
-// (c) Copyright Microsoft Corporation.
+// (c) Copyright Microsoft Corporation.
 // This source is subject to the Microsoft Public License (Ms-PL).
 // Please see http://go.microsoft.com/fwlink/?LinkID=131993 for details.
 // All other rights reserved.
@@ -50,11 +50,6 @@ namespace Avalonia.Controls
             var value = (IDataTemplate)e.NewValue;
         }
 
-        public DataGridTemplateColumn()
-        {
-            // IsReadOnly = true;
-        }
-        
         protected override IControl GenerateElement(DataGridCell cell, object dataItem)
         {
             if(CellTemplate != null)