Remove three unused declaration. (#26699)
Summary:
`frac()` in `Vec256<int{16,32,64}_t>` is not overridden.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/26699
Differential Revision: D17549502
Pulled By: soumith
fbshipit-source-id: 87c65286032bfc88c447ec4eef1e3ebc73da5d27
