net/mosquitto: Fix sample mosquitto.conf for 2.0.22

mosquitto now complains if cafile is set but certfile and keyfile
are not:

1755534419: Error: cafile specified without certfile and keyfile.
/usr/local/etc/rc.d/mosquitto: WARNING: failed to start mosquitto

Avoid installing a invalid sample config; leave cafile unset.

Also avoid hardcoded /usr/local

Reported by: Steffen Dirkwinkel
Pull Request: https://github.com/freebsd/freebsd-ports/pull/417