Remove unused Ruby code not used in almost a year
This was a missed spot in the 6e43d9f3 (Remove Ruby RefService, 2020-02-06), which was the last user of this "delete_branch" function.
This was a missed spot in the 6e43d9f3 (Remove Ruby RefService, 2020-02-06), which was the last user of this "delete_branch" function.