Remove special case for aarch64 static vs. PIC code in iOS kernel code.

I added this check a while back but then made a note to myself that it
should be completely unnecessary since iOS always uses PIC code-gen for
aarch64. Since I could never come up with any reason why it would be
necessary, I'm just going to remove it and we'll see if anything breaks.
rdar://problem/13627985

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@223097 91177308-0d34-0410-b5e6-96231b3b80d8