ref: Remove unused FindRefName RPC
The FindRefName RPC hasn't been used by Rails since fa0b4321 (Added
validation to AfterCreateService, 2020-03-02), which is more than a year
ago. While the stub to call this RPC still existed in Rails until
ec8ca3cdb44 (repository: Remove #ref_name_for_sha
, 2021-08-18), it
didn't have any callers at all anymore.
Let's remove this RPC call in Gitaly.