qa/workunits/rest/test.py: do not test 'osd thrash'

This wreaks havoc on our QA because it marks osds up and down and then 
immediately after that we try to scrub and some osds are still down.  I think
it is not worth futzing with here since this is a debug/dev operation and is
covered by the CLI test.

Signed-off-by: Sage Weil <sage@inktank.com>