ARM: drop check for triple that's no longer used.

Early attempts to support AAPCS bare metal MachO targets based the decision on
the CPU being compiled for. This was not a particularly great idea and we've
got a better option now, but this check remained.

No functional change for any target we care about.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219767 91177308-0d34-0410-b5e6-96231b3b80d8