qa/tasks: fix flake8 findings in ceph.py and radosbench.py

The cephx merge (96335615c1b) left four unused local variables in
qa/tasks/ceph.py (key_prune/key_rotate/task) and an f-string without
placeholders plus an unused exception binding in qa/tasks/radosbench.py,
so run-tox-qa (flake8 --select=F,E9) fails on every PR.  No functional
change.

Fixes: 96335615c1bfdd598412d8768be09323d299ba33
Signed-off-by: David Galloway <david.galloway@ibm.com>
Assisted-by: Claude Fable 5 (claude-fable-5)