it's less that the u is silent, and more that the pronunciation of it is not QUITE desu or deskaah, japanese has different phonological backgrounds than english, and therefore is only imperfectly modeled by the latin alphabet, and our phonological uinderstanding of it either desu, or deskaah, will make you sound like you have an accent. but either is servicible. deskaah is closer. as the missing phonological element is really very subtle.