Get rid of node-exporter-scripts

The current solution of dumping executable scripts in /etc/prometheus/node-exporter-scripts doesn't really do much on top of "just dump something in /etc/cron.d", it shares all its same issues, so we might just as well get rid of the "custom" solution in favor of standard cron scripts redirected to /var/lib/prometheus/node-exporter.

Maybe we can just ship a wrapper script to set up the output redirection properly, simplifying even further the /etc/cron.d entries.