Remove warning to only use swift with keystone v2

The warning to only use swift with the keystone v2 API
should be removed now that a fix has been merged to make
swift aware of v3 domains.

Fix in swift is https://review.openstack.org/#/c/86430

Change-Id: Iac172a48513dee25aa2d0830257d46f25f0d5648
Related-bug: #1299146
Merge "Remove warning to only use swift with keystone v2"