Remove useless check for ThreadModel==Single in ARMISelLowering. NFC.

ThreadModel::Single is already handled already by ARMPassConfig adding
LowerAtomicPass to the pass list, which lowers all atomics to non-atomic
ops and deletes fences.

So by the time we get to ISel, there's no atomic fences left, so they
don't need special handling.

llvm-svn: 265178