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()`.

llvm-svn: 218820