Remove feature flag for FindAllRemoteBranches

Acceptance testing done through:
https://gitlab.com/gitlab-org/gitaly/issues/1312

Relatively short AT period, but given its not a hard RPC, nor anything
funky is going on, I felt that this was decent enough to remove the
feature flag.

Closes https://gitlab.com/gitlab-org/gitaly/issues/1243

lib/gitlab/git/repository_mirroring.rb

@@ -2,34 +2,7 @@ module Gitlab
   module Git
     module RepositoryMirroring
       def remote_branches(remote_name)
-        gitaly_migrate(:ref_find_all_remote_branches) do |is_enabled|
-          if is_enabled
         gitaly_ref_client.remote_branches(remote_name)
-          else
-            Gitlab::GitalyClient::StorageSettings.allow_disk_access do
-              rugged_remote_branches(remote_name)
-            end
-          end
-        end
-      end
-
-      private
-
-      def rugged_remote_branches(remote_name)
-        branches = []
-
-        rugged.references.each("refs/remotes/#{remote_name}/*").map do |ref|
-          name = ref.name.sub(%r{\Arefs/remotes/#{remote_name}/}, '')
-
-          begin
-            target_commit = Gitlab::Git::Commit.find(self, ref.target.oid)
-            branches << Gitlab::Git::Branch.new(self, name, ref.target, target_commit)
-          rescue Rugged::ReferenceError
-            # Omit invalid branch
-          end
-        end
-
-        branches
       end
     end
   end