Fix: notification: client with uid != trigger uid assert on gid check

Fix: notification: client with uid != trigger uid assert on gid check

The gid was removed from the trigger object since gid is not an exposed
control measure anywhere in lttng except for the "tracing" group and
lttng-sessiond to client communication.

Also based on this:

 /*
 * As for privilieged users, they can register triggers against the objects of
 * other users. They can then subscribe to the notifications associated to their
 * triggers. Privilieged users _can't_ subscribe to the notifications of
 * triggers owned by other users; they must create their own triggers.
 * /

This is why the current check is not bypassed if the notification client
have a UID == 0 (root).

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I690ea355b0b1ab0bfe82b7db47275876afe622f2