Remove Nova v3 API endpoint

Nova v3 API has disappeared in Juno cycle, and we don't test the API
now on the gate since If63dcdb2d05aa0fab0b6848a1248b6678f1ee9ad .
This patch removes the endpoint of Nova v3 API.

Change-Id: I85f87b37558a15d1eaaa781b02fec5b02bd2ab44
Merge "Remove Nova v3 API endpoint"