removed reference to DocFX (#19713)

DocFX support for this tags is incomplete and there is confusion on that since it is listed here, implying it is supported. See [DocFX issue ](dotnet/docfx#2328 (comment)) for more details.

Author: smaillet-ms

docs/csharp/programming-guide/xmldoc/include.md

@@ -69,7 +69,7 @@ The summary for this other type.
 
 ## Program output
 
-The following output is generated when you compile the Test and Test2 classes with the following command line: `-doc:DocFileName.xml.` In Visual Studio, you specify the XML doc comments option in the Build pane of the Project Designer. When the C# compiler sees the `<include>` tag, it searches for documentation comments in *xml_include_tag.doc* instead of the current source file. The compiler then generates *DocFileName.xml*, and this is the file that is consumed by documentation tools such as [DocFX](https://dotnet.github.io/docfx/) and [Sandcastle](https://github.com/EWSoftware/SHFB) to produce the final documentation.  
+The following output is generated when you compile the Test and Test2 classes with the following command line: `-doc:DocFileName.xml.` In Visual Studio, you specify the XML doc comments option in the Build pane of the Project Designer. When the C# compiler sees the `<include>` tag, it searches for documentation comments in *xml_include_tag.doc* instead of the current source file. The compiler then generates *DocFileName.xml*, and this is the file that is consumed by documentation tools such as [Sandcastle](https://github.com/EWSoftware/SHFB) to produce the final documentation.  
 
 ```xml
 <?xml version="1.0"?>