Remove valid epic check from EE::Issues::ExportCsvService

What does this MR do and why?

Related to #365043 (closed).

This MR removes the line that checks if the epic_issue link is valid when exporting issues data.

This check can be expensive and is no longer needed after running the migration to clean invalid records.

See !76686 (comment 963988189) for more context.

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Related to #339514