blockfreq: Remove BlockMass*BlockMass

Since `BlockMass` is an implementation detail and there are no current
users of this, delete `BlockMass::operator*=(BlockMass)`.  I might need
this when I try to strip out `UnsignedFloat`, but I can pull it back in
at that point.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207546 91177308-0d34-0410-b5e6-96231b3b80d8