Removed unused service endpoints

Previous versions—and specific customers—used extensions which relied on the underlying **OnTopic** data model, and were similar in nature to e.g. `Topics.Json.aspx`, and were thus grouped together under `/Client`—but which were never actually needed by **OnTopic Editor**. Given that this project is now obsolete and no longer being supported, there's no real value to retaining these moving forward. All customers that had needs for these end points are no longer depending on them, and if subsequent customers need them, we'd obviously want to reevaluate those needs based on more recent libraries (such as our new **ASP.NET Core 3.1** adapter).

Author: JeremyCaney

OnTopic.Editor.Web/Client/Sitemap.xml.aspx

@@ -55,9 +55,7 @@
     </Reference>
   </ItemGroup>
   <ItemGroup>
-    <Content Include="Client\Sitemap.xml.aspx" />
     <Content Include="Client\Topics.Json.aspx" />
-    <Content Include="Client\Topics.xml.aspx" />
     <Content Include="Common\Controls\Boolean.ascx" />
     <Content Include="Common\Controls\DateTimeSelector.ascx" />
     <Content Include="Common\Controls\File.ascx" />