## Description 

External consensus has not been updated or actively used for awhile as
far as we know. Removing them can simplify the Narwhal codebase and make
future changes easier.

#12946

## Test Plan 

CI
Narwhal nightly:
https://github.com/MystenLabs/sui/actions/runs/5522777741/jobs/10072838309

---
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### Type of Change (Check all that apply)

- [ ] protocol change
- [ ] user-visible impact
- [ ] breaking change for a client SDKs
- [ ] breaking change for FNs (FN binary must upgrade)
- [ ] breaking change for validators or node operators (must upgrade
binaries)
- [ ] breaking change for on-chain data layout
- [ ] necessitate either a data wipe or data migration

### Release notes