blockfreq: Defer to BranchProbability::scale() (again)

Change `BlockFrequency` to defer to `BranchProbability::scale()` and
`BranchProbability::scaleByInverse()`.

This removes `BlockFrequency::scale()` from its API (and drops the
ability to see the remainder), but the only user was the unit tests.  If
some code in the future needs an API that exposes the remainder, we can
add something to `BranchProbability`, but I find that unlikely.