Remove feature gate for ListNewCommits

Introduced by https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20087,
this has been tested on .com now, and is stable.

Closes https://gitlab.com/gitlab-org/gitaly/issues/1286
Closes https://gitlab.com/gitlab-org/gitaly/issues/1233

lib/gitlab/git/repository.rb

@@ -366,18 +366,9 @@ module Gitlab
         end
       end
 
-      # Gitaly migration: https://gitlab.com/gitlab-org/gitaly/issues/1233
       def new_commits(newrev)
-        gitaly_migrate(:new_commits) do |is_enabled|
-          if is_enabled
+        wrapped_gitaly_errors do
           gitaly_ref_client.list_new_commits(newrev)
-          else
-            refs = Gitlab::GitalyClient::StorageSettings.allow_disk_access do
-              rev_list(including: newrev, excluding: :all).split("\n").map(&:strip)
-            end
-
-            Gitlab::Git::Commit.batch_by_oid(self, refs)
-          end
         end
       end
 

spec/models/repository_spec.rb

@@ -296,7 +296,6 @@ describe Repository do
   end
 
   describe '#new_commits' do
-    shared_examples 'finding unreferenced commits' do
     set(:project) { create(:project, :repository) }
     let(:repository) { project.repository }
 
@@ -324,15 +323,6 @@ describe Repository do
     end
   end
 
-    context 'when Gitaly handles the request' do
-      it_behaves_like 'finding unreferenced commits'
-    end
-
-    context 'when Gitaly is disabled', :disable_gitaly do
-      it_behaves_like 'finding unreferenced commits'
-    end
-  end
-
   describe '#commits_by' do
     set(:project) { create(:project, :repository) }