Remove usages of Contract.Requires.

src/Avalonia.Controls/Calendar/CalendarItem.cs

@@ -921,7 +921,6 @@ namespace Avalonia.Controls.Primitives
                     if (b.IsEnabled && !b.IsBlackout)
                     {
                         DateTime selectedDate = (DateTime)b.DataContext!;
-                        Contract.Requires<ArgumentNullException>(selectedDate != null);
                         _isMouseLeftButtonDown = true;
                         // null check is added for unit tests
                         if (e != null)

src/Avalonia.Controls/Grid.cs

@@ -599,7 +599,7 @@ namespace Avalonia.Controls
         {
             double value = 0.0;
 
-            Contract.Requires<NullReferenceException>(_data != null);
+            Debug.Assert(_data != null);
 
             //  actual value calculations require structure to be up-to-date
             if (!ColumnDefinitionsDirty)
@@ -621,7 +621,7 @@ namespace Avalonia.Controls
         {
             double value = 0.0;
 
-            Contract.Requires<NullReferenceException>(_data != null);
+            Debug.Assert(_data != null);
 
             //  actual value calculations require structure to be up-to-date
             if (!RowDefinitionsDirty)