MachO: Remove an unused variable from processSection

The local variable `cfi` became dead in r220730 when it's use was
obviated; it was replaced with a call to read32.

No functionality change intended.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@221412 91177308-0d34-0410-b5e6-96231b3b80d8