Revert "Disallow system notes for closed issuables"

This reverts commit 07717744.

app/services/system_note_service.rb

@@ -385,7 +385,6 @@ module SystemNoteService
   # Returns Boolean
   def cross_reference_disallowed?(noteable, mentioner)
     return true if noteable.is_a?(ExternalIssue) && !noteable.project.jira_tracker_active?
-    return true if noteable.is_a?(Issuable) && (noteable.try(:closed?) || noteable.try(:merged?))
     return false unless mentioner.is_a?(MergeRequest)
     return false unless noteable.is_a?(Commit)

spec/services/system_note_service_spec.rb

@@ -418,45 +418,6 @@ describe SystemNoteService, services: true do
           to be_truthy
       end
     end
-
-    context 'when noteable is an Issue' do
-      let(:issue) { create(:issue, project: project) }
-
-      it 'is truthy when issue is closed' do
-        issue.close
-
-        expect(described_class.cross_reference_disallowed?(issue, project.commit)).
-          to be_truthy
-      end
-
-      it 'is falsey when issue is open' do
-        expect(described_class.cross_reference_disallowed?(issue, project.commit)).
-          to be_falsy
-      end
-    end
-
-    context 'when noteable is a Merge Request' do
-      let(:merge_request) { create(:merge_request, :simple, source_project: project) }
-
-      it 'is truthy when merge request is closed' do
-        allow(merge_request).to receive(:closed?).and_return(:true)
-
-        expect(described_class.cross_reference_disallowed?(merge_request, project.commit)).
-          to be_truthy
-      end
-
-      it 'is truthy when merge request is merged' do
-        allow(merge_request).to receive(:closed?).and_return(:true)
-
-        expect(described_class.cross_reference_disallowed?(merge_request, project.commit)).
-          to be_truthy
-      end
-
-      it 'is falsey when merge request is open' do
-        expect(described_class.cross_reference_disallowed?(merge_request, project.commit)).
-          to be_falsy
-      end
-    end
   end
   describe '.cross_reference_exists?' do