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

(cherry picked from commit aa48259f337100e79933d660fec8856371f761ed)