DIBuilder: Remove dead code

I neglected to update `DIBuilder::createPieceExpression()` in r218797,
which I noticed while rebasing a patch for PR17891.  On closer
inspection, it looks like dead code.

If there are any downstream users of this, you should transition to the
more general `createExpression()`.  Or, we can add this back, but then
it should just forward to `createExpression()`.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218820 91177308-0d34-0410-b5e6-96231b3b80d8