Remove the unused EditingTimeCompilationRequest websocket request

### Task Summary

`EditingTimeCompilationRequest` is a websocket request type that the frontend no longer sends and the backend no longer handles.

**Introduced by** #2195 (2023-11-09, "Enhance frontend error reporting"), which had the editor send the logical plan over the websocket to collect static errors.

**Usage removed by** #3178 (2024-12-29, "Remove cache checker in logical plan"), which deleted the handler that took it. Compilation moved to the HTTP `workflow-compiling-service`, and `workflow-compiling.service.ts` still documents the switch: *"receive static errors (previously done by sending EditingTimeCompilationRequest ...)"*.

Dead for about a year and a half. What remains is the case class, its `@JsonSubTypes` registration, spec entries, and a frontend type-map entry — no sender and no handler.

Pure deletion, no behaviour change: −39 lines.

### Task Type

- [x] Refactor / Cleanup