jail_solaris: Retire mgt_param.[gu]id usage

This fields were never set in the first place so they went away in #3888.

We don't have SunOS coverage on Github so I noticed it after the facts.
I did look at the Solaris jail but somehow missed that those fields were
used there as well. Chances are that the deleted statements never ran in
the first place, otherwise the assertions would have triggered.

If the solaris jail should set[gu]id(2) as part of its privileges drop,
it should probably grow new sub-options similar to the ones in the unix
jail.

Refs #3888

bin/varnishd/mgt/mgt_jail_solaris.c

@@ -447,10 +447,6 @@ static void
 vjs_setuid(void)
 {
 	if (priv_ineffect(PRIV_PROC_SETID)) {
-		if (getgid() != mgt_param.gid)
-			XXXAZ(setgid(mgt_param.gid));
-		if (getuid() != mgt_param.uid)
-			XXXAZ(setuid(mgt_param.uid));
 		AZ(setppriv(PRIV_OFF, PRIV_EFFECTIVE, vjs_proc_setid));
 		AZ(setppriv(PRIV_OFF, PRIV_PERMITTED, vjs_proc_setid));
 	} else {