Remove redundant comment and cop disable

What does this MR do?

After !18002 (merged) was merged, we could use comments on the opening block line, thus the commentary in fast_hash_serializer.rb is no longer relevant.

Also, removing unnecessary cop disable in base_exporter.rb.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content