Remove useless lttng_metadata probe

We don't use this event anymore since we write the metadata directly
into the ring buffer, no need for an external event. This probe was
the only one in the lttng-probe-lttng module, so we can get rid of
this module as well.

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>