remove test markup

Author: ButchersBoy

MainDemo.Wpf/TextFields.xaml

@@ -115,7 +115,6 @@
         <TextBox Grid.Row="4" Grid.Column="1" materialDesign:HintAssist.Hint="Floating Hint" 
                  Style="{StaticResource MaterialDesignFloatingHintTextBox}" />
         <ComboBox Grid.Row="4" Grid.Column="3" materialDesign:HintAssist.Hint="OS" Style="{StaticResource MaterialDesignFloatingHintComboBox}">
-            <ComboBoxItem />
             <ComboBoxItem>Android</ComboBoxItem>
             <ComboBoxItem>iOS</ComboBoxItem>
             <ComboBoxItem>Linux</ComboBoxItem>