Improve default route on macOS (#1680)

re: issue #1088
and probably: https://discuss.zerotier.com/t/default-route-issue-osx-monterey-m1/6974

// current zerotier.
// allow default adds two sets of routes.
```
netstat -rnfinet | grep "/1\|default" | sort
0/1                10.2.0.2           UGScg        feth4823
0/1                192.168.82.1       UGScIg            en7
128.0/1            10.2.0.2           UGSc         feth4823
128.0/1            192.168.82.1       UGScI             en7
default            192.168.82.1       UGScg             en7
```

Then, something chaotic happens eventually, and networking stops working.

// after patch
```
netstat -rnfinet | grep "/1\|default" | sort
0/1                10.2.0.2           UGScg        feth4823
128.0/1            10.2.0.2           UGSc         feth4823
default            192.168.82.1       UGScg             en7
```

After the change, I can still:
- use default route
- route to other subnets

I tested on high sierra through monterey
and on freebsd13.1

osdep/ManagedRoute.cpp

@@ -439,76 +439,6 @@ bool ManagedRoute::sync()
 
 #ifdef __BSD__ // ------------------------------------------------------------
 
-	if (_device[0]) {
-		bool haveDevice = false;
-		struct ifaddrs *ifa = (struct ifaddrs *)0;
-		if (!getifaddrs(&ifa)) {
-			struct ifaddrs *p = ifa;
-			while (p) {
-				if ((p->ifa_name)&&(!strcmp(_device, p->ifa_name))) {
-					haveDevice = true;
-					break;
-				}
-				p = p->ifa_next;
-			}
-			freeifaddrs(ifa);
-		}
-		if (!haveDevice)
-			return false;
-	}
-
-	// Find lowest metric system route that this route should override (if any)
-	InetAddress newSystemVia;
-	char newSystemDevice[128];
-	newSystemDevice[0] = (char)0;
-	int systemMetric = 9999999;
-	std::vector<_RTE> rtes(_getRTEs(_target,false));
-	for(std::vector<_RTE>::iterator r(rtes.begin());r!=rtes.end();++r) {
-		if (r->via) {
-			if ( ((!newSystemVia)||(r->metric < systemMetric)) && (strcmp(r->device,_device) != 0) ) {
-				newSystemVia = r->via;
-				Utils::scopy(newSystemDevice,sizeof(newSystemDevice),r->device);
-				systemMetric = r->metric;
-			}
-		}
-	}
-
-	// Get device corresponding to route if we don't have that already
-	if ((newSystemVia)&&(!newSystemDevice[0])) {
-		rtes = _getRTEs(newSystemVia,true);
-		for(std::vector<_RTE>::iterator r(rtes.begin());r!=rtes.end();++r) {
-			if ( (r->device[0]) && (strcmp(r->device,_device) != 0) ) {
-				Utils::scopy(newSystemDevice,sizeof(newSystemDevice),r->device);
-				break;
-			}
-		}
-	}
-	if (!newSystemDevice[0])
-		newSystemVia.zero();
-
-	// Shadow system route if it exists, also delete any obsolete shadows
-	// and replace them with the new state. sync() is called periodically to
-	// allow us to do that if underlying connectivity changes.
-	if ((_systemVia != newSystemVia)||(strcmp(_systemDevice,newSystemDevice) != 0)) {
-		if (_systemVia) {
-			_routeCmd("delete",leftt,_systemVia,_systemDevice,(const char *)0);
-			if (rightt)
-				_routeCmd("delete",rightt,_systemVia,_systemDevice,(const char *)0);
-		}
-
-		_systemVia = newSystemVia;
-		Utils::scopy(_systemDevice,sizeof(_systemDevice),newSystemDevice);
-
-		if (_systemVia) {
-			_routeCmd("add",leftt,_systemVia,_systemDevice,(const char *)0);
-			//_routeCmd("change",leftt,_systemVia,_systemDevice,(const char *)0);
-			if (rightt) {
-				_routeCmd("add",rightt,_systemVia,_systemDevice,(const char *)0);
-				//_routeCmd("change",rightt,_systemVia,_systemDevice,(const char *)0);
-			}
-		}
-	}
-
 	//if (!_applied.count(leftt)) {
 		_applied[leftt] = !_via;
 		//_routeCmd("delete",leftt,_via,(const char *)0,(_via) ? (const char *)0 : _device);