Remove legacy Sentinel from common/api_handlers.py

Starts only after all 23 per-domain stories are merged.

## Scope

|Target|Change|
|---|---|
|`common/api_handlers.py` L43-49|Delete `class Sentinel` and `SENTINEL`|

## Done when

- Zero imports of `api_handlers.Sentinel` / `SENTINEL` across the repository
- `./backend.ai mgr api dump-openapi` output contains no `Sentinel` reference (currently 78 fields across 25 schemas)

## Next

- BA-7101 (#13299): remove `exclude_none=True` at `client/v2/base_client.py` L179, L206, L459 so the CLI/SDK can send `null` to clear a nullable field

JIRA Issue: BA-7767

Wait until all 23 per-domain stories are merged, then inspect common/api_handlers.py lines 43-49 and check repository-wide references to api_handlers.Sentinel and SENTINEL. Run ./backend.ai mgr api dump-openapi; done means no such imports remain and the output contains no Sentinel reference.

Written by the indexing model from the issue text.