MB-100: remove unused imports in swaprebalance.py

Change-Id: I994828b2398c3760f8b8048f1ac1c623c409bff0
Reviewed-on: http://review.couchbase.org/23988
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>