Yes, of course! All you need is katsuobushi. At a fundamental level, making Dashi with only katsuobushi is precisely the same as making it with kombu and katsuobushi. The difference is in the amount necessary, but this difference is slight. Kombu contains glutamates, which are the source of the umami in Dashi, so it's natural to assume that katsuobushi supplies them too. But the truth is that the umami contained in good-quality katsuobushi mostly comes from inosinates.