checks: Always enable batched computation of commits

With 156ce433 (checks: Implement infrastructure for bulk diff checks,
2021-07-29), we have implemented batched loading of all commits for
access checks in contrast to loading commits for each change. This both
enables us to do a single RPC call to fetch all changes, and furthermore
it allows us to use the quarantine directory to enumerate new commits
instead of doing a revision walk. Together, these are a big performance
improvement based on how many references a repository has: for
gitlab-org/gitlab, the time to compute new commits went down from 19
seconds to about 12 milliseconds.

Remove the feature flag of this important performance optimization. The
feature flag had been enabled in production on August 16th and can thus
be deemed stable.

Changelog: performance

config/feature_flags/development/changes_batch_commits.yml

----
-name: changes_batch_commits
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/66731
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/336992
-milestone: '14.2'
-type: development
-group: group::gitaly
-default_enabled: false

lib/gitlab/checks/changes_access.rb

@@ -81,7 +81,7 @@ module Gitlab
       def single_access_checks!
         # Iterate over all changes to find if user allowed all of them to be applied
         changes.each do |change|
-          commits = Gitlab::Lazy.new { commits_for(change[:newrev]) } if Feature.enabled?(:changes_batch_commits)
+          commits = Gitlab::Lazy.new { commits_for(change[:newrev]) }
 
           # If user does not have access to make at least one change, cancel all
           # push by allowing the exception to bubble up

spec/lib/gitlab/checks/changes_access_spec.rb

@@ -8,7 +8,6 @@ RSpec.describe Gitlab::Checks::ChangesAccess do
   subject { changes_access }
 
   describe '#validate!' do
-    shared_examples '#validate!' do
     before do
       allow(project).to receive(:lfs_enabled?).and_return(true)
     end
@@ -41,23 +40,6 @@ RSpec.describe Gitlab::Checks::ChangesAccess do
     end
   end
 
-    context 'with batched commits enabled' do
-      before do
-        stub_feature_flags(changes_batch_commits: true)
-      end
-
-      it_behaves_like '#validate!'
-    end
-
-    context 'with batched commits disabled' do
-      before do
-        stub_feature_flags(changes_batch_commits: false)
-      end
-
-      it_behaves_like '#validate!'
-    end
-  end
-
   describe '#commits' do
     it 'calls #new_commits' do
       expect(project.repository).to receive(:new_commits).and_call_original