trim dead sabaki enginesyncer undo code

[Terkwood]

this is not used by the BUGOUT undo system

if (
        !this.state.dirty &&
        sharedHistoryLength > 0 &&
        undoLength < sharedHistoryLength &&
        (this.commands.includes("undo") || undoLength === 0)
      ) {
        // Undo until shared history is reached, then play out rest

        promises = [
          ...[...Array(undoLength)].map(() => () =>
            controller.sendCommand({ name: "undo" }).then((r) => !r.error)
          ),
          ...promises.slice(sharedHistoryLength),
        ];
      }

[Terkwood]

Resolved by #491. The if branch was causing that issue.