RHEL-9 doesn't ship flake8. Note that it was already disabled for
%check anyway, but we also have an internal beaker test that runs
test.sh where we would have to disable it too, at which point it's just
easier to do that in test.sh itself.
While at it, remove all traces of flake8 from the spec file as well, to
avoid confusion in the future.
Related: #1929210
