remove unused function
make cron module work on solaris

Cron on solaris do not take the same
set of option than vixie cron on linux, and
among the biggest difference, root cannot set
the crontab of a user directly from a file. Thus the
use of su to run the crontab command. Fix issue #4648
Merge pull request #4754 from mscherer/fix_4648

Fix 4648, cron module is not working on solaris