DwarfUnit: Remove some misleading no-op code introduced in r204162.

Post commit review feedback from Manman called this out, but it looks
like it slipped through the cracks.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209611 91177308-0d34-0410-b5e6-96231b3b80d8