chore(hybridcloud) Remove dynamic default_flush on ApiToken

[markstory]

We can't easily make ApiToken flush async because of read-after write usage in cell endpoints. Remove the rollout option & tests for this experiment.

Refs #105264

[github-actions]

Backend Test Failures

Failures on c222247 in this run:

tests/sentry/models/test_apitoken.py::ApiTokenInternalIntegrationTest::test_async_replication_schedules_drain_task — log

[gw0] linux -- Python 3.13.1 /home/runner/work/sentry/sentry/.venv/bin/python3
tests/sentry/models/test_apitoken.py:360: in test_async_replication_schedules_drain_task
    assert mock_drain_task.called
E   AssertionError: assert False
E    +  where False = <MagicMock name='delay' id='139812395279632'>.called

tests/sentry/models/test_apitoken.py::ApiTokenInternalIntegrationTest::test_multiple_tokens_use_different_shards — log

[gw1] linux -- Python 3.13.1 /home/runner/work/sentry/sentry/.venv/bin/python3
tests/sentry/models/test_apitoken.py:390: in test_multiple_tokens_use_different_shards
    assert token_1_outboxes.exists()
E   assert False
E    +  where False = exists()
E    +    where exists = <BaseQuerySet []>.exists