bt2: Adapt test_stream.py and make it pass

bt2: Adapt test_stream.py and make it pass

This patch updates test_stream.py and stream.py.  It remove everything
related to equality, copy and deepcopy, as in the previous patches.

Since we don't want to share code between the CTF writer and the
trace-ir objects, the _StreamBase class is removed, and ctf_writer.py is
updated just so import still works.

Change-Id: I72e80694e0c8b401a86ce23d94b6e064afb08ac2
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1292
Tested-by: jenkins
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>