Remove the commented-out MockKillWorkerResource

### Task Summary

`MockKillWorkerResource.scala` is a file whose every non-blank line after the ASF header is a comment — the commented-out remains of a JAX-RS `/kill/worker` endpoint. `TexeraWebApplication` still carries the matching commented-out registration line, `// environment.jersey().register(classOf[MockKillWorkerResource])`.

**Introduced by** the initial amber import, commit `41a8a92017` (2020-08-20) — predates the PR workflow — which also registered it live with Jersey.

**Disabled by** #1000 (`4a64935c35`, 2021-01-30, "Migrate all control handling logics to AsyncRPC and cleanup Amber"), which commented out the registration line and the whole file body in the same diff. Nothing has compiled from the file since.

Registered live for about five months, 2020-08-20 → 2021-01-30, then commented out for the five and a half years since. The commented registration line goes with the file.

Pure deletion, no behaviour change: −40 lines.

### Task Type

- [x] Refactor / Cleanup
- [ ] DevOps / Deployment / CI
- [ ] Testing / QA
- [ ] Documentation
- [ ] Performance
- [ ] Other