Remove `ci_job_heartbeats_runner` feature flag

This is no longer
being used

config/feature_flags/development/ci_job_heartbeats_runner.yml

----
-name: ci_job_heartbeats_runner
-introduced_by_url: 
-rollout_issue_url: 
-group: 
-type: development
-default_enabled: true

lib/api/helpers/runner.rb

@@ -51,9 +51,7 @@ module API
           job_forbidden!(job, 'Job is not running') unless job.running?
         end
 
-        if Gitlab::Ci::Features.job_heartbeats_runner?(job.project)
-          job.runner&.heartbeat(get_runner_ip)
-        end
+        job.runner&.heartbeat(get_runner_ip)
 
         job
       end

lib/gitlab/ci/features.rb

@@ -10,10 +10,6 @@ module Gitlab
         ::Feature.enabled?(:ci_artifacts_exclude, default_enabled: true)
       end
 
-      def self.job_heartbeats_runner?(project)
-        ::Feature.enabled?(:ci_job_heartbeats_runner, project, default_enabled: true)
-      end
-
       def self.instance_variables_ui_enabled?
         ::Feature.enabled?(:ci_instance_variables_ui, default_enabled: true)
       end