Simplify subscriptionlist repository fallback data fetching

There has been a cron job that has updated this repository outside of this code since 2015, it doesn't run that often, and the files are not that large, so we can most likely tolerate the possible race condition between reads and writes (just like we do elsewhere, e.g. notifications/parser.py since 2021)

This change will allow us to change the version control system on that checkout from hg to git (as the source of truth has been in git for years as well, we don't need the hg mirror)