Nope....
Honestly, they are already greatly conditioned, and they drop the bulk of the weight by just working out and eating even healthier than they are. I guarentee no boxer is losing fifteen pounds in a day or whatever it was. I have seen people lose six to seven, but fifteen is exaggeration. It becomes a way of life, it is not something you do, it is all you do. Boxing has the day before weighins too IIRC. That is what gets you. Generally, you go and binge after, plus all the liquids you take in. I am sure each boxer has his own routine, and I'd love to see who is dropping twenty pounds in a day.