Remove API client from LightlySSL (#1939)

* refactor!: remove lightly.api, lightly.openapi_generated, lightly.active_learning

* feat(cli)!: remove lightly-download and lightly-serve

* refactor: drop API-only dependencies and tooling config

* test: drop api_workflow tests and mocks

* ci: drop api-deps-only workflow and version-check env vars

* docs: remove API client documentation

* test: remove stale token override

* fix: add deprecated api import error

* docs: point README to LightlyStudio per review (#1939)

* fix: drop redundant sentence from lightly.api error message

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

* test: assert on "removed" in lightly.api import error

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>