tools/workqueue/wq_dump.py: Clean up code and drop duplicate information

- Factor out wq_type_str()

- Improve formatting so that it adapts to actual field widths.

- Drop duplicate information from "Workqueue -> rescuer" section. If
  anything, we should add more rescuer-specific info - e.g. the number of
  work items rescued.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Juri Lelli <juri.lelli@redhat.com>