net/3proxy: fix defect in its startup script

The startup script of 3proxy supplied with the port
unnecessary overrides default start_cmd constructed
with rc.subr procedures. This breaks, for example,
the following setting in /etc/rc.conf:

threeproxy_fib="1"

This and similar <servicename>_SETTINGS are ignored due to such an override.
Fix it.

PR: 293937
Maintainer timeout: 3 weeks