bt2: rename object's own BT class property to `cls`

bt2: rename object's own BT class property to `cls`

Having `mein_event.event_class` is redundant; what we really want is
`mein_event.class`, but `class` is a reserved word, so use `cls`.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ib1f0c91ca37e91b7704ca61fb7380bb79cb48927
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1394
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>