This operation seems vestigial, as it was added to the code when stack.sh
did not have a robust cleanup procedure. These days, unstack.sh does destroy
all bridges, therefore during subsequent stack.sh runs (or even initially, from
a clean environment), the flush operation has become superfluous.
Its removal has also been deemeded necessary to enable certain multi-node
cloud deployments, like the one available in OpenStack infra [1].
[1] https://review.openstack.org/#/c/158525/
Change-Id: I6b4e5b82958e6d29dd450f1c4c9513f6a9e5053a
c2dc95add6