onionperf: No longer need to download tpf files

The OnionPerf module still has residual code related to the download of *.tpf files, which are no longer produced by modern OnionPerf. This code could be removed, and in the process might make the JSON downloading code that remains more robust.

Relevant code: https://gitlab.torproject.org/tpo/metrics/collector/-/blob/master/src/main/java/org/torproject/metrics/collector/onionperf/OnionPerfDownloader.java