Replace the link_window request by a set_parent request since that is
all link_window is used for at this point.
Get rid of the WIN_LinkWindow/UnlinkWindow functions.

diff --git a/server/request.h b/server/request.h
index 7ce7aa1..5c8ffe2 100644
--- a/server/request.h
+++ b/server/request.h
@@ -240,11 +240,11 @@
 DECL_HANDLER(disconnect_named_pipe);
 DECL_HANDLER(get_named_pipe_info);
 DECL_HANDLER(create_window);
-DECL_HANDLER(link_window);
 DECL_HANDLER(destroy_window);
 DECL_HANDLER(set_window_owner);
 DECL_HANDLER(get_window_info);
 DECL_HANDLER(set_window_info);
+DECL_HANDLER(set_parent);
 DECL_HANDLER(get_window_parents);
 DECL_HANDLER(get_window_children);
 DECL_HANDLER(get_window_children_from_point);
@@ -431,11 +431,11 @@
     (req_handler)req_disconnect_named_pipe,
     (req_handler)req_get_named_pipe_info,
     (req_handler)req_create_window,
-    (req_handler)req_link_window,
     (req_handler)req_destroy_window,
     (req_handler)req_set_window_owner,
     (req_handler)req_get_window_info,
     (req_handler)req_set_window_info,
+    (req_handler)req_set_parent,
     (req_handler)req_get_window_parents,
     (req_handler)req_get_window_children,
     (req_handler)req_get_window_children_from_point,