mod_pubsub: Remove 'dag' node plugin documentation

Commit c9d3beb9eb removed the 'dag'
node/nodetree plugins.

src/mod_pubsub.erl

@@ -4294,11 +4294,7 @@ mod_doc() ->
 		      "nodes will have the default configuration, and this "
 		      "can not be changed. Using 'virtual' nodetree requires "
 		      "to start from a clean database, it will not work if "
-		      "you used the default 'tree' nodetree before."),
-		   ?T("- 'dag' nodetree provides experimental support for "
-		      "PubSub Collection Nodes (XEP-0248). In that case you "
-		      "should also add 'dag' node plugin as default, for "
-		      "example: 'plugins: [flat,pep]'")]}},
+		      "you used the default 'tree' nodetree before.")]}},
 	   {pep_mapping,
 	    #{value => "List of Key:Value",
 	      desc =>