Remove unnecessary has_many relations from EE::Issue

app/models/ee/issue.rb

 module EE
   module Issue
-    extend ActiveSupport::Concern
-
-    prepended do
-      has_many :referenced_issue_links, class_name: 'IssueLink', foreign_key: :source_id
-      has_many :referred_by_issue_links, class_name: 'IssueLink', foreign_key: :target_id
-      has_many :referenced_issues, through: :referenced_issue_links, source: :target
-      has_many :referred_by_issues, through: :referred_by_issue_links, source: :source
-    end
-
     # override
     def check_for_spam?
       author.support_bot? || super

spec/lib/gitlab/import_export/all_models.yml

@@ -18,10 +18,6 @@ issues:
 - metrics
 - timelogs
 - issue_assignees
-- referenced_issue_links
-- referred_by_issue_links
-- referenced_issues
-- referred_by_issues
 events:
 - author
 - project