Fix: sessiond: agent: lazy creation of agent on removal

When unregistering a trigger that was previously registered, the
agent corresponding to the trigger's domain will already exist. It is
superfluous to handle the case where it doesn't exist and it would only
hide an internal error.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ifc6d14b74586af8423a70b3466e0a2c00491d3c3