bt2: remove unused imports

bt2: remove unused imports

I ran flake8 and removed unused import warnings in all files, except
__init__.py, where the imports are present but not used in that file on
purpose.

Change-Id: Ib422a25b042eb1ba0a68e5c254947b5cfe3b7398
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1886
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>