Address nits on lib/placement for extracted placement

Earlier review [1] suggested some cleanups which have been
done here:

* Removing a redundant call from cleanup_placement
* Fixing a typo in a comment

[1] https://review.openstack.org/#/c/600162/15/lib/placement

Change-Id: I8abd2f02b123c6c1937c026ff13eb4e600de3202
1 file changed