security/ca_root_nss: adjust pkg-plist to address leftover directory

The port creates /etc/ssl directory with the default option, but until now,
did not remove it upon deinstallation. While technically this requires
a revbump, rebuilding this port to fix a cleanup step would cause a
tremendous amount of fallout and it's not worth the pain IMO.

PR: 213121
Approved by: feld (ports-secteam)