Remove `ci_pipeline_open_merge_requests` FF

This feature is stable and no longer needs
a feature flag.

app/models/ci/pipeline.rb

@@ -803,7 +803,7 @@ module Ci
           variables.concat(merge_request.predefined_variables)
         end
 
-        if Gitlab::Ci::Features.pipeline_open_merge_requests?(project) && open_merge_requests_refs.any?
+        if open_merge_requests_refs.any?
           variables.append(key: 'CI_OPEN_MERGE_REQUESTS', value: open_merge_requests_refs.join(','))
         end
 

config/feature_flags/development/ci_pipeline_open_merge_requests.yml

----
-name: ci_pipeline_open_merge_requests
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/38673
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/292727
-milestone: '13.7'
-group: group::memory
-type: development
-default_enabled: true

lib/gitlab/ci/features.rb

@@ -55,10 +55,6 @@ module Gitlab
         ::Feature.enabled?(:ci_trace_log_invalid_chunks, project, type: :ops, default_enabled: false)
       end
 
-      def self.pipeline_open_merge_requests?(project)
-        ::Feature.enabled?(:ci_pipeline_open_merge_requests, project, default_enabled: true)
-      end
-
       def self.ci_pipeline_editor_page_enabled?(project)
         ::Feature.enabled?(:ci_pipeline_editor_page, project, default_enabled: :yaml)
       end