Cleanup migration code and remove feature flag after two successful rekeying and roster adoptions

After deployment, validate
1. the next candidate roster set is keyed successfully before the freeze.
2. On startup of the next software upgrade, the candidate roster and new ledger id are adopted.
3. before freeze, the next candidate roster is keyed properly, transferring the ledger id to the next roster.
4. On startup of the following software upgrade, the candidate roster with the transferred ledger id is adopted without problem.
5. Merge the migration cleanup code to development to remove the TSS-Ledger-ID feature flag.

Start by locating the migration cleanup code and the TSS-Ledger-ID feature flag, then review the deployment and software-upgrade sequence described in the issue. Done means validating both successful rekeying and roster-adoption cycles and merging the cleanup that removes the feature flag.

Written by the indexing model from the issue text.