Las Vegas Sun: Itchy eyes and swollen sinuses — these are the signs of spring in Las Vegas. And parts of summer. Oh, and a bit of fall, too. It wasn’t always like this. There was a time when people suffering from respiratory illnesses flocked to the Mojave Desert to escape allergens. But then their neighbors planted olive and mulberry trees, and what was once a haven became hell for a growing number of residents. If climatologists and botanists are right, it wasn’t all the …