ports.7: Do not mention deprecated WITH_SSP_PORTS and WITH_GHOSTSCRIPT_VER

As mentioned in this special status report[1] from EuroBSDCon 2013,
WITH_SSP_PORTS was relevant in the FreeBSD 9 and FreeBSD 10 days.
Nowadays, -fstack-protector is set by default. Relevant knobs and variables
are documented in ports/Mk/bsd.port.mk.

WITH_GHOSTSCRIPT_VER was part of USE_GHOSTSCRIPT, which is deprecated.

[1]: https://www.freebsd.org/news/status/report-2013-09-devsummit.html#Ports-and-Packages

Reviewed by: bcr, mat, tobik
Approved by: bcr (doc)
Approved by: krion (mentor, implicit), mat (mentor, implicit)
Differential Revision: https://reviews.freebsd.org/D18056