Ignore top-level "status" field in responses.

For compatibility with existing tests (which have no way to see the HTTP
status itself, so can't be easily modified), the "status" field in JSON
responses is populated with the actual HTTP status.

Rationale: We'd like to get rid of this field in Crab, since it offers
no value (one has to check the HTTP status first to know whether to
expect the response to be parsable anyway) and it requires either
redundant code or awkward filters to support.

(This has nothing to do with the "status" sub-field of the contact put response, which will remain after the proposed response changes.)

Does anyone know of a system that depends on this field?

Systems known not to care about this field:
- [x] Switchboard
- [x] Finn (with this PR)
- [x] FCX
- [ ] Webapp
- [ ] iOS app
- [ ] Deduplication
- [ ] Card Reader
- [ ] Anything else?