Remove the empty ExecutionStatusEnum.scala

### Task Summary

`amber/src/main/scala/org/apache/texera/web/model/websocket/event/ExecutionStatusEnum.scala` is an 18-line file that holds the ASF license header and nothing else — no code, and nothing in the tree references the name. `git grep -n ExecutionStatusEnum` returns nothing at all; it survives only as a filename. Flagged as a follow-up in the #8396 review.

**Introduced by** #1351 (2021-10-27, "Detach workflow execution from frontend with reconnection support") as a `sealed trait ExecutionStatusEnum` with a hand-written Jackson serializer/deserializer, carried by `WorkflowStateEvent`; #1423 (2022-02-01, "Track metadata information about workflow executions") reshaped it into a `sealed abstract class` carrying a `name` and a DB status `code`.

**Emptied by** #1387 (2022-02-10, "Web server workflow-related state refactoring"), which moved execution state to the protobuf `WorkflowAggregatedState`, changed `WorkflowStateEvent.state` to a `String`, and left this file as a single blank line instead of deleting it. #3415 (2025-05-03, "Add ASF header and RAT CI") then stamped the license header onto the empty file, which is all it has held since.

Empty for about four and a half years. Like the `MockKillWorkerResource` removal in #8396, a file with zero compiled lines.

Pure deletion, no behaviour change: −18 lines.

### Task Type

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