Remove valid epic check

Checking for each epic validatiy in
Issues::ExportCsvService is no longer needed
after migration that deleted invalida records.
Given it is an expensive check, we need to remove
it

Changelog: fixed
EE: true

ee/app/services/ee/issues/export_csv_service.rb

@@ -26,9 +26,7 @@ def header_to_value_hash
       def epic_issue_safe(attribute)
         lambda do |issue|
           epic = issue.epic
-
           next if epic.nil?
-          next unless issue.epic_issue.valid? # TODO This check can be removed with https://gitlab.com/gitlab-org/gitlab/-/issues/339514
 
           epic[attribute]
         end

ee/spec/services/issues/export_csv_service_spec.rb

@@ -62,20 +62,4 @@ def csv
       it_behaves_like 'including issues with epics'
     end
   end
-
-  context 'when epic issue is not valid' do
-    let(:epic) { create(:epic, group: group) }
-
-    before do
-      create(:epic_issue, issue: issue, epic: epic)
-
-      allow_next_instance_of(EpicIssue) do |epic_issue|
-        allow(epic_issue).to receive(:valid?).and_return(false)
-      end
-    end
-
-    it 'does not include epics information' do
-      expect(csv[0]).not_to have_key('Epic ID')
-    end
-  end
 end