Currently, we may clear a CDC generation's data from
CDC_GENERATIONS_V3 if it is not the last committed generation
and it is at least 24 hours old (according to the topology
coordinator's clock). However, after allowing writes to the
previous CDC generations, this condition became incorrect. We
might clear data of a generation that could still be written to.
The new solution is to clear data of the generations that
finished operating more than 24 hours ago. The rationale behind
it is in the new comment in
`topology_coordinator:clean_obsolete_cdc_generations`.
The previous solution used the clean-up candidate. After
introducing `committed_cdc_generations`, it became unneeded.
The last obsolete generation can be computed in
`topology_coordinator:clean_obsolete_cdc_generations`. Therefore,
we remove all the code that handles the clean-up candidate.
After changing how we clear CDC generations' data,
`test_current_cdc_generation_is_not_removed` became obsolete.
The tested feature is not present in the code anymore.
`test_dependency_on_timestamps` became the only test case covering
the CDC generation's data clearing. We adjust it after the changes.
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