ref(cells): update deletions code to reference cells not regions

[lynnagara]

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[github-actions]

Backend Test Failures

Failures on 4bbdedc in this run:

tests/sentry/taskworker/test_config.py::test_all_instrumented_tasks_registered — log

tests/sentry/taskworker/test_config.py:120: in test_all_instrumented_tasks_registered
    raise AssertionError(
E   AssertionError: Found 1 module(s) with @instrumented_task that are NOT registered in TASKWORKER_IMPORTS.
E   These tasks will not be discovered by the taskworker in production!
E   
E   Missing modules:
E     - sentry.workflow_engine.tasks.cleanup
E   
E   Add these to TASKWORKER_IMPORTS in src/sentry/conf/server.py