don't set HasReliableSymbolDifference for ELF.

It is only used in MachObjectWriter.cpp. Another leftover from early days
of ELF in MC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200895 91177308-0d34-0410-b5e6-96231b3b80d8