[APInt] Remove return value from tcFullMultiply.

The description says it returns the number of words needed to represent the results. But the way it was coded it always returns (lhsWords + rhsWords) or (lhsWords + rhsWords - 1). But the result could be even smaller than that and it wouldn't tell you.

No one uses the result today so rather than try to fix it, just remove it.

llvm-svn: 302551