To move it, use:
But you are absolutely free to remove it. It is there by default to help with Leaflet adoption (because many devs don't care if it's present), but it's not required by any means.
In other news, I've adjusted the default prefix in master recently — got rid of the "powered by", now it's just a Leaflet link with a "|" separator — much less obtrusive. What do you think guys?