2d23820ee11678d567c75f94c40011ed9f0e274f refactor: remove dead branches in `SingletonClusterImpl` (Lőrinc)
Pull request description:
Found during review: [cluster mempool: control/optimize TxGraph memory usage](https://github.com/bitcoin/bitcoin/pull/33157#discussion_r2423058928)
### Fixes
`SplitAll()` always calls `ApplyRemovals()` first, for a singleton, it empties the cluster, therefore any `SingletonClusterImpl` passed to `Split()` must be empty.
`TxGraphImpl::ApplyDependencies()` first merges each dependency group and asserts the group has at least one dependency. Since `parent` != `child`, `TxGraphImpl::Merge()` upgrades the merge target to `GenericClusterImpl`, therefore the `ApplyDependencies()` is never dispatched to `SingletonClusterImpl`.
### Coverage proof:
* https://maflcko.github.io/b-c-cov/fuzz.coverage/src/txgraph.cpp.gcov.html#L1446
* https://storage.googleapis.com/oss-fuzz-coverage/bitcoin-core/reports/20251103/linux/src/bitcoin-core/src/txgraph.cpp.html#L1446
ACKs for top commit:
instagibbs:
ACK 2d23820ee11678d567c75f94c40011ed9f0e274f
sipa:
ACK 2d23820ee11678d567c75f94c40011ed9f0e274f
Tree-SHA512: 5135913206c800d5344df61c6654f00917cb85567bc5b821576c7891805cf7689bf47968434a06517d09183dadfefc257d24c42b55a7b99486a4c9b11fc523af
2b9c351198