security/sssd: disable po4a

Disable po4a. It is not an explicit build dependency, nor should
it be. There is no need to use it to rebuild man pages with the
current (1.16.5) distribution tarball. And using po4a (currently
0.68) somehow fails to create man pages for some locales.

PR: 267415
Approved by: portmgr (maintainer timeout, 4+ months)