While police continue to sweep heroin and crack fiends from the Boston Common and Public Garden, Mayor Thomas M. Menino is preparing to launch an anti-litter campaign for all the city's parks. Even with police trying to shut down the startling open-air drug abuse in and around the two premier greenspaces, some of it observed in broad daylight by the Herald in recent weeks, Menino said it was the right time to launch the litter crusade. Cleaner parks might discourage drug abuse, he said. ``Cleaning the parks sends a message that we're not going to tolerate it,'' he said yesterday....