Do not reference deskutils/cal from cal.1.

The ports version of cal is an abandonware so in order to minimize the
potential bit rot of our documentation let's not mention it at all.
Interested users are going to find suitable alternatives anyway on their
own.

Reported by: bapt
Approved by: bapt (src)
Approved by: krion (mentor, implicit), mat (mentor, implicit)
Differential Revision: https://reviews.freebsd.org/D19492