python: fix all 'is assigned to but never used' warnings

python: fix all 'is assigned to but never used' warnings

Fix all instances of this warning (code F841) found in the repo by
flake8.  In one instance (in test_plugin.py), I added an assertion to
use the variable.  In all others, I considered the variable not useful
and removed it.

Change-Id: I95c282a610d0e3b8adf7cee40ce5229744e8ebc0
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reported-by: flake8
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1978
Tested-by: jenkins <jenkins@lttng.org>