Backbulbs don't need to be removed when repotting. Matter of fact, if you leave them on, you'll give the plant a better chance of acclimating to its new environment, and it will grow new roots faster. The only plant material you should get rid of is anything dead, dying, or infected--roots and backbulbs that won't contribute to a plant's vigor, and which, if left on, can spread infection to healthy growths. If you cut away backbulbs, be sure to either spray the cut with pruning sealer or dip it in fungicide so that the moist environment beneath the media won't contribute to any kind of invasive infection.