bt2: Adapt test_event_class.py and make it pass

bt2: Adapt test_event_class.py and make it pass

This patch updates event_class.py to be more in line with the current
API and updates test_event_class.py accordingly.

Everything related to equality, copy and deepcopy is removed.  An event
class is always created from an existing stream class, so it is not
longer possible to create an event class out of thin air.  We only
support passing parameters to the event class when creating it, not
assigning them afterwards.

Change-Id: I2f1ad9f98f25e3e2dfdea511a1410529d014745b
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1296
Tested-by: jenkins
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>