Adding context to an event is no longer possible

Adding context to an event is no longer possible

The UST tracer does not allow to add context to an event so we've
removed that.

It is completely removed from the lttng UI so it's not possible either
to add one to the kernel. The "event_name" of lttng_add_context is now
ignored.

Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>