applet: don't call nm_secret_agent_register() explicitly for AppletAgent

We create the secret agent with 'auto-register' true. Hence
explicitly registering results in a race due to a bug in libnm.

The libnm bug was fixed by commit 86ffea8004980b9ab931d3f172e89fe192af6cd0 .

https://bugzilla.gnome.org/show_bug.cgi?id=727923

Signed-off-by: Thomas Haller <thaller@redhat.com>
(cherry picked from commit d540a4e56589c1c6e6ba0bbe7558b1f7734c9879)