After the update of JIRA Agile from 6.3.9.1 to 6.6.0, the "Rank" field was re-worked into a new Lexo rank field and the original field was re-named to "Rank (Obsolete)" as explained in the upgrade notes at : https://confluence.atlassian.com/display/AGILE/JIRA+Agile+6.4+Upgrade+Notes.

Although it is safe to remove this field later, but the customer needs to go through the process of removing the field through the database. The upgrade process should automatically take care of deleting the field.