x11/xscreensaver: Remove PAM option

Upstream ./configure now fails with --without-pam if PAM is found on the
build system with this message:

Your system seems to have PAM, but PAM is not being used.
That is probably not going to work out well.

We have PAM in base and can't build base without it any more
(WITHOUT_PAM is deprecated), so let's just remove that port option and
always build xscreensaver with PAM (previously enabled by default).

With PAM enforced, support for shadow passwords is not needed. Forcing
it to off, we can avoid installing xscreensaver-auth suid root.

PR: 264361
Approved by: x11 (cy, manu), tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D36425