Remove reference to dead domain

Author: WhyNotHugo

docs/when.rst

@@ -50,7 +50,6 @@ program chosen:
 
 * Such a setup doesn't work at all with smartphones. Vdirsyncer, on the other
   hand, synchronizes with CardDAV/CalDAV servers, which can be accessed with
-  e.g. DAVx⁵_ or the apps by dmfs_.
+  e.g. DAVx⁵_ or other apps bundled with smartphones.
 
 .. _DAVx⁵: https://www.davx5.com/
-.. _dmfs: https://dmfs.org/