Add a 'force' option for non-interactive crontab removal

Add a '-f' option to force crontab '-r' to be non-interactive.

Submitted by: Sam Gwydir <sam at samgwydir.com>
Reviewed by: me, wblock (previous version)
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D8815