[client] Clean up bsd routes independently of the state file (#4688)

client/internal/routemanager/systemops/flush_nonbsd.go

@@ -0,0 +1,8 @@
+//go:build !((darwin && !ios) || dragonfly || freebsd || netbsd || openbsd)
+
+package systemops
+
+// FlushMarkedRoutes is a no-op on non-BSD platforms.
+func (r *SysOps) FlushMarkedRoutes() error {
+	return nil
+}