Remove deploy_tokens_api feature flag

Remove deploy_tokens_api feature flag which
has been enabled since 12.9.

config/feature_flags/development/deploy_tokens_api.yml

----
-name: deploy_tokens_api
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/25219
-rollout_issue_url: 
-milestone: '12.9'
-type: development
-group: group::package
-default_enabled: true

lib/api/deploy_tokens.rb

@@ -28,8 +28,6 @@ module API
       use :pagination
     end
     get 'deploy_tokens' do
-      service_unavailable! unless Feature.enabled?(:deploy_tokens_api, default_enabled: true)
-
       authenticated_as_admin!
 
       present paginate(DeployToken.all), with: Entities::DeployToken
@@ -39,10 +37,6 @@ module API
       requires :id, type: String, desc: 'The ID of a project'
     end
     resource :projects, requirements: API::NAMESPACE_OR_PROJECT_REQUIREMENTS do
-      before do
-        service_unavailable! unless Feature.enabled?(:deploy_tokens_api, user_project, default_enabled: true)
-      end
-
       params do
         use :pagination
       end
@@ -102,10 +96,6 @@ module API
       requires :id, type: String, desc: 'The ID of a group'
     end
     resource :groups, requirements: API::NAMESPACE_OR_PROJECT_REQUIREMENTS do
-      before do
-        service_unavailable! unless Feature.enabled?(:deploy_tokens_api, user_group, default_enabled: true)
-      end
-
       params do
         use :pagination
       end

spec/requests/api/deploy_tokens_spec.rb

@@ -10,24 +10,12 @@ RSpec.describe API::DeployTokens do
   let!(:deploy_token) { create(:deploy_token, projects: [project]) }
   let!(:group_deploy_token) { create(:deploy_token, :group, groups: [group]) }
 
-  shared_examples 'with feature flag disabled' do
-    context 'disabled feature flag' do
-      before do
-        stub_feature_flags(deploy_tokens_api: false)
-      end
-
-      it { is_expected.to have_gitlab_http_status(:service_unavailable) }
-    end
-  end
-
   describe 'GET /deploy_tokens' do
     subject do
       get api('/deploy_tokens', user)
       response
     end
 
-    it_behaves_like 'with feature flag disabled'
-
     context 'when unauthenticated' do
       let(:user) { nil }
 
@@ -81,8 +69,6 @@ RSpec.describe API::DeployTokens do
         project.add_maintainer(user)
       end
 
-      it_behaves_like 'with feature flag disabled'
-
       it { is_expected.to have_gitlab_http_status(:ok) }
 
       it 'returns all deploy tokens for the project' do
@@ -128,8 +114,6 @@ RSpec.describe API::DeployTokens do
         group.add_maintainer(user)
       end
 
-      it_behaves_like 'with feature flag disabled'
-
       it { is_expected.to have_gitlab_http_status(:ok) }
 
       it 'returns all deploy tokens for the group' do