Hannibal was trying to add credibility to his self-defence story
If the statue was found to have fingerprints on it, Hannibal would have to explain how and why he hit Tobias in the back of the head with it, ultimately delivering the killing blow.
The FBI might wonder:
If Hannibal was behind Tobias long enough to be able to grab the statue, pick it up, and hit him, why didn't Hannibal opt for a non-lethal option such as pinning him down or applying a choke hold. Tobias would have to have been distracted or incapacitated to allow Hannibal the time to hit him - which portrays Hannibal as more of a murderer than someone who acted in self-defence. (Note, I'm not a lawyer or legal expert!)
By removing the fingerprints, Hannibal could perhaps argue that Tobais was knocked back onto the statue during the scuffle, or that it landed on him.