Fix: Only list kernel enablers when listing events

lttng list would list kernel enablers and individual enabled
syscalls which would result in a confusing output.

For instance, enabling the "open" syscall with a filter would
result in a list output displaying both the enabler,
the syscall's name + its filter, and the syscall on its own,
without any mention of an enabled filter.

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