Remove UserControl style.

It is styled as a ContentControl.

src/Perspex.Themes.Default/DefaultTheme.paml

@@ -24,6 +24,5 @@
   <StyleInclude Source="resource://application/Perspex.Themes.Default/Perspex.Themes.Default.ToggleButton.paml"/>
   <StyleInclude Source="resource://application/Perspex.Themes.Default/Perspex.Themes.Default.TreeView.paml"/>
   <StyleInclude Source="resource://application/Perspex.Themes.Default/Perspex.Themes.Default.TreeViewItem.paml"/>
-  <StyleInclude Source="resource://application/Perspex.Themes.Default/Perspex.Themes.Default.UserControl.paml"/>
   <StyleInclude Source="resource://application/Perspex.Themes.Default/Perspex.Themes.Default.Window.paml"/>
 </Styles>

src/Perspex.Themes.Default/Perspex.Themes.Default.csproj

@@ -140,6 +140,9 @@
     <EmbeddedResource Include="ProgressBar.paml">
       <SubType>Designer</SubType>
     </EmbeddedResource>
+    <EmbeddedResource Include="UserControl.paml">
+      <SubType>Designer</SubType>
+    </EmbeddedResource>
     <None Include="packages.config" />
     <EmbeddedResource Include="PopupRoot.paml">
       <SubType>Designer</SubType>

src/Perspex.Themes.Default/UserControl.paml

@@ -1,13 +0,0 @@
-<Style xmlns="https://github.com/perspex" Selector="UserControl">
-  <Setter Property="Template">
-    <ControlTemplate>
-      <Border Background="{TemplateBinding Background}"
-              BorderBrush="{TemplateBinding BorderBrush}"
-              BorderThickness="{TemplateBinding BorderThickness}">
-        <ContentPresenter Name="PART_ContentPresenter" 
-                          Content="{TemplateBinding Content}" 
-                          Margin="{TemplateBinding Padding}"/>
-      </Border>
-    </ControlTemplate>
-  </Setter>
-</Style>