Deprecate stats_solr and any of its code

[cdrini]

A subtask of #5143
A continuation of #4426

1. @cdrini Gzip the following directories:
   • /var/lib/solr
   • /etc/solr
     time tar -czf /1/solr-backups/stats-solr-$(date +%Y-%m-%d).tar.gz /var/lib/solr /etc/solr
2. @cdrini Confirm gzip can be rehydrated / write docs
3. Upload ol-dev1:/opt/stats-solr-backup to a private IA item
   • Coordinate w/ @mekarpeles about the right access-restricted tags to use
   • https://github.com/internetarchive/olsystem/pull/179
4. Stop solr's tomcat server ; confirm no one is using it in a weird indirect way -- sudo /etc/init.d/tomcat6 stop
   • sudo /etc/init.d/tomcat6 status

   • • Tomcat servlet engine is not running.

   • Generates a Sentry error once every 30 minutes. https://sentry.archive.org/organizations/ia-ux/issues/356628
5. Stop the process_stats cron jobs from running, by commenting them out on ol-home and ol-home0, and restart the crons on each.
   • https://github.com/internetarchive/olsystem/pull/180 -- Done in the cron-jobs Docker container.
   • No cron jobs run on ol-home
   • Cron jobs on on-home0 run in the openlibrary-cron-jobs-1 Docker container.
6. Remove process_stats.py, loanstats.py and anything else. There should be no refs to stats_solr. Remove stats_solr as part of our migration to ol-home0 #7902
7. Remove the config from olsystem, and the cron lines in olsystem.
   • internetarchive/olsystem#180
   • internetarchive/olsystem#183