message: validation, doc: remove TODO comment
  It would make for sense for the TODO to be done in PR 17487
  (or noted in the review feedback for a follow-up),
  no need to continue maintaining the TODO in the codebase.
diff: @@ -4787,7 +4787,6 @@ static void FlushSnapshotToDisk(CCoinsViewCache& coins_cache, bool snapshot_load
  coins_cache.DynamicMemoryUsage() / (1000 * 1000)),
  BCLog::LogFlags::ALL);
  - // TODO: if #17487 is merged, add erase=false here if snapshot is loaded, for better performance.
  coins_cache.Flush();
  }
  