Remove one change from https://reviews.llvm.org/D115431

The change to ArchSpec::SetArchitecture that was setting the
ObjectFile of a mach-o binary to llvm::Triple::MachO.  It's not
necessary for my patch, and it changes the output of image list -t
causing TestUniversal.py to fail on x86_64 systems.  The bots
turned up the failure, I was developing and testing this on
an Apple Silicon mac.