Remove the unused TupleUtils.json2tuple

### Task Summary

`TupleUtils.json2tuple` has no production caller. Its sibling `tuple2json` is live (`ExecutionResultService` uses it) and stays.

**Introduced by** #1328 (2021-09-08, "[Operator Caching Step 3] add operator cache backend new files and config changes"), and genuinely used — the cache read path called `val newTuple = json2tuple(line)` when rehydrating cached tuples.

**Usage removed by** #3111 (2024-11-26, "Update amber to depend on sub projects"), which deleted that call site while splitting amber into sub-projects. From that commit on, the only references are the definition and its tests.

Dead for about two years. Removing it also frees three imports that only it used: `AttributeTypeUtils.{inferSchemaFromRows, parseField}`, `JSONUtils.{JSONToMap, objectMapper}`, and `ArrayBuffer`.

Pure deletion, no behaviour change: −114 lines.

### Task Type

- [x] Refactor / Cleanup