Remove trackers from BackgroundThread

[owbone]

We no longer need to maintain the trackers here because:

1. we're no longer writing them to the .litbin files,
2. they are updated from the backend.

The only thing they are actually used for is resuming experiments and keeping track of the last step, but can use the new lit_logger_service_get_logger_metrics_summary endpoint to do that now, so let's just keep track of the last step per metric rather than the full tracker information.