Stop generating .tpf files

As of last week, CollecTor, metrics-lib, and metrics-web all know how to process OnionPerf's .json files. Unless anything else uses .tpf files we can now stop generating those.

Suggested process:

• Discuss any remaining uses of .tpf files where .json files are not yet supported.
• Stop generating .tpf files on deployed instances op-{hk,nl,us}2, and if somebody complains, re-generate them from existing log files until they have switched.
• Remove the code that generates .tpf files from OnionPerf.