In the past, we used the `SR-Risk` and `C-Protocol-Critical` labels to require specific engineers to review PRs touching sensitive code:

- `SR-Risk` would tag @maurelian whenever legacy mainnet smart contracts were touched.
- `C-Protocol-Critical` would tag @tynes or @smartcontracts whenever smart contracts or `l2geth` were touched.

Since launching Bedrock, the `SR-Risk` label was removed from the `labeler` workflow, so it was never being applied. `C-Protocol-Critical` was still being applied, however it had no effect apart from blocking Mergify and pinging Mark/Kelvin unnecessarily since it is `CODEOWNERS` and not the label that prevents merge.

Given that we've started using `CODEOWNERS` and the fact that the labels were broken, this PR removes these legacy labels altogether.