ports-mgmt/synth: disable hanging process watchdog due to scons

After setting the group process ID and then execv'ing into the
ports build target, any port using scons just locks up immediately
without any kind of message. I assume it's a bug in scons but it
will take a while to resolve (probably requires a testcase case and
submitting upstream). Since it caused a significant regression and
since normally users shouldn't hit hung processes often, I'm disabling
the feature for now.