Drop jira_use_first_ref_by_oid feature flag

We made this feature flag true by default in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/73510. It has been
working well in production for several weeks now.

Changelog: changed

app/models/integrations/jira.rb

@@ -303,11 +303,7 @@ module Integrations
     private
 
     def branch_name(commit)
-      if Feature.enabled?(:jira_use_first_ref_by_oid, project, default_enabled: :yaml)
-        commit.first_ref_by_oid(project.repository)
-      else
-        commit.ref_names(project.repository).first
-      end
+      commit.first_ref_by_oid(project.repository)
     end
 
     def server_info

config/feature_flags/development/jira_use_first_ref_by_oid.yml

----
-name: jira_use_first_ref_by_oid
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/72739
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/343585
-milestone: '14.5'
-type: development
-group: group::integrations
-default_enabled: true

spec/models/integrations/jira_spec.rb

@@ -937,18 +937,6 @@ RSpec.describe Integrations::Jira do
         end
       end
 
-      context 'with jira_use_first_ref_by_oid feature flag disabled' do
-        before do
-          stub_feature_flags(jira_use_first_ref_by_oid: false)
-        end
-
-        it 'creates a comment and remote link on Jira' do
-          expect(subject).to eq(success_message)
-          expect(WebMock).to have_requested(:post, comment_url).with(body: comment_body).once
-          expect(WebMock).to have_requested(:post, remote_link_url).once
-        end
-      end
-
       it 'tracks usage' do
         expect(Gitlab::UsageDataCounters::HLLRedisCounter)
           .to receive(:track_event)