Stop using custom DefaultContextMenu

Because a custom context menu was being used, the spell check context
menu items weren't appearing. Our custom DefaultContextMenu was showing
Cut, Copy and Paste (the same as the normal default).

Author: jcansdale

src/GitHub.UI/Assets/Controls.xaml

@@ -19,13 +19,6 @@
     Styles for standard windows controls
     -->
 
-     <ContextMenu x:Key="DefaultContextMenu">
-        <MenuItem Header="Cut" Command="ApplicationCommands.Cut"/>
-        <MenuItem Header="Copy" Command="ApplicationCommands.Copy"/>
-        <MenuItem Header="Paste" Command="ApplicationCommands.Paste"/>
-    </ContextMenu>
-    <!--End ContextMenu-->
-    
     <!-- ProgressBar -->
     <Style x:Key="GitHubProgressBar" TargetType="{x:Type ui:GitHubProgressBar}">
         <Setter Property="Foreground" Value="#999999" />

src/GitHub.UI/Assets/Controls/FilterTextBox.xaml

@@ -14,7 +14,6 @@
         <Setter Property="ScrollViewer.PanningMode" Value="VerticalFirst"/>
         <Setter Property="Stylus.IsFlicksEnabled" Value="False"/>
         <Setter Property="Padding" Value="3,3,18,3" />
-        <Setter Property="ContextMenu" Value="{DynamicResource DefaultContextMenu}" />
         <Setter Property="Template">
             <Setter.Value>
                 <ControlTemplate TargetType="{x:Type ui:FilterTextBox}">

src/GitHub.UI/Assets/TextBlocks.xaml

@@ -15,7 +15,6 @@
         <Setter Property="BorderThickness" Value="1" />
         <Setter Property="ToolTipService.ShowDuration" Value="30000" />
         <Setter Property="ToolTipService.ShowOnDisabled" Value="True" />
-        <Setter Property="ContextMenu" Value="{DynamicResource DefaultContextMenu}" />
         <Setter Property="Template">
             <Setter.Value>
                 <ControlTemplate TargetType="{x:Type TextBox}">