adjust version definition for v3 to be only json

the version definition for the v3 API still publicized xml as
a supported mime type. We should not do that.

Part of bp remove-v3-xml-api

Change-Id: Iba87b7ee05f0a89a3f7051d4fea8a91635d7778d
Merge "adjust version definition for v3 to be only json"