Remove the unused search-by-operators workflow endpoint

### Task Summary

`WorkflowResource.searchWorkflowByOperator` (`GET /workflow/search-by-operators`) has no client. Workflow search in the UI goes to the unified `/dashboard/search` resource; nothing calls this path.

**Provenance.** Introduced by #1611 (2022-08-07, "Search Workflows Feature") and genuinely used at the time — the frontend held `WORKFLOW_OPERATOR_URL = WORKFLOW_BASE_URL + "/search-by-operators"` and called it. #2038 (2023-07-07, "Allow users to cascade the sharing") deleted that constant when search moved to the dashboard resource; from that commit on, the only file mentioning the path is the backend resource itself. Dead for about three years.

The frontend's `searchByOperators` test in `user-workflow.component.spec.ts` is a name coincidence — it drives the filter UI (`component.filters.operators`), which queries `/dashboard/search`, not this endpoint.

Removing the method also frees `Condition` and `noCondition`, which no other method in `WorkflowResource` uses.

Pure deletion, no behaviour change: −66 lines.

### Task Type

- [x] Refactor / Cleanup