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Remove the old Enum ResponseAction and use net_traits::FetchResponseMsg instead.
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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #13717 (github issue number if applicable).
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- [ ] There are tests for these changes OR
- [X] These changes do not require tests because @KiChjang said so.
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