services: Remove lsh service from %DESKTOP-SERVICES.

Suggested by Mark H Weaver and others
at <http://lists.gnu.org/archive/html/guix-devel/2015-06/msg00227.html>.

* gnu/services/desktop.scm (%desktop-services): Remove call to 'lsh-service'.
* doc/guix.texi (Desktop Services): Adjust accordingly.