Is it permissible to work in a restaurant where alcohol is served to customers and the job requires cleaning up after them and disposing of alcohol bottles, given the difficulty of finding a purely halal job in America?

If your work does not involve any assistance in the drinking of alcohol, then there is no sin upon you for carrying its bottles to discard and destroy them. You must keep away from places where alcohol is consumed, and not show any contentment with those who consume it, for Allah the Almighty says: (And He has already sent down to you in the Book that when you hear the verses of Allah disbelieved in and ridiculed, then do not sit with them until they engage in another conversation. Indeed, you would then be like them). If you are unable to denounce them, then you must leave them and seek work where you do not see alcohol or its drinkers, as that is more fitting.