Remove feature flag disable_joins_upstream_downstream_projects

This has been enabled on production for 2 days already per
https://gitlab.com/gitlab-org/gitlab/-/issues/341791 without incident.

Changelog: other

config/feature_flags/development/disable_joins_upstream_downstream_projects.yml

----
-name: disable_joins_upstream_downstream_projects
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/71247
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/341791
-milestone: '14.4'
-type: development
-group: group::sharding
-default_enabled: false

ee/app/models/ee/project.rb

@@ -93,9 +93,9 @@ module EE
       has_many :project_aliases
 
       has_many :upstream_project_subscriptions, class_name: 'Ci::Subscriptions::Project', foreign_key: :downstream_project_id, inverse_of: :downstream_project
-      has_many :upstream_projects, class_name: 'Project', through: :upstream_project_subscriptions, source: :upstream_project, disable_joins: -> { ::Feature.enabled?(:disable_joins_upstream_downstream_projects, default_enabled: :yaml) }
+      has_many :upstream_projects, class_name: 'Project', through: :upstream_project_subscriptions, source: :upstream_project, disable_joins: true
       has_many :downstream_project_subscriptions, class_name: 'Ci::Subscriptions::Project', foreign_key: :upstream_project_id, inverse_of: :upstream_project
-      has_many :downstream_projects, class_name: 'Project', through: :downstream_project_subscriptions, source: :downstream_project, disable_joins: -> { ::Feature.enabled?(:disable_joins_upstream_downstream_projects, default_enabled: :yaml) }
+      has_many :downstream_projects, class_name: 'Project', through: :downstream_project_subscriptions, source: :downstream_project, disable_joins: true
 
       has_many :sourced_pipelines, class_name: 'Ci::Sources::Project', foreign_key: :source_project_id