etc/crontab: Remove dead code

This is now running through the swh scheduler

etc/crontab

-SHELL=/bin/bash
-GHLISTER_ROOT=/home/zack/src/swh-lister-github
-
-# m h  dom mon dow   command
-  0 8  *   *   *     PYTHONPATH=$GHLISTER_ROOT $GHLISTER_ROOT/bin/ghlister catchup >> ~/.cache/swh/lister-github/$(date +\%Y\%m\%d).log 2>&1