FindRefName RPC now mandatory

Has never been in OPT_OUT, dispite being ages old. The time where
feature flags are still set by env vars.

Closes https://gitlab.com/gitlab-org/gitaly/issues/221

lib/gitlab/git/repository.rb

@@ -613,17 +613,7 @@ module Gitlab
       def ref_name_for_sha(ref_path, sha)
         raise ArgumentError, "sha can't be empty" unless sha.present?
 
-        gitaly_migrate(:find_ref_name) do |is_enabled|
-          if is_enabled
         gitaly_ref_client.find_ref_name(sha, ref_path)
-          else
-            args = %W(for-each-ref --count=1 #{ref_path} --contains #{sha})
-
-            # Not found -> ["", 0]
-            # Found -> ["b8d95eb4969eefacb0a58f6a28f6803f8070e7b9 commit\trefs/environments/production/77\n", 0]
-            run_git(args).first.split.last
-          end
-        end
       end
 
       # Get refs hash which key is is the commit id