periodic.conf: remove long deprecated security_daily_compat_var()

This function is documented to be gone in after 11. Time to remove this
compat shim.

PR: 275296
Reviewed by: jrm (mentor)
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D44796

Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

Move all periodic related config and scripts to usr.sbin/periodic/

This makes pkgbase easier by tagging these as CONFS so they are properly
tagged as config files.

Approved by: will (mentor)
Sponsored by: Essen Hackathon
Differential Revision: https://reviews.freebsd.org/D16553