Remove UseThumbBacktraces. Just check if subtarget is darwin.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@73734 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Target/ARM/ARMSubtarget.cpp b/lib/Target/ARM/ARMSubtarget.cpp
index a978380..7ac7b49 100644
--- a/lib/Target/ARM/ARMSubtarget.cpp
+++ b/lib/Target/ARM/ARMSubtarget.cpp
@@ -24,7 +24,6 @@
   , ARMFPUType(None)
   , IsThumb(isThumb)
   , ThumbMode(Thumb1)
-  , UseThumbBacktraces(false)
   , IsR9Reserved(false)
   , stackAlignment(4)
   , CPUString("generic")
@@ -83,8 +82,6 @@
   if (isAAPCS_ABI())
     stackAlignment = 8;
 
-  if (isTargetDarwin()) {
-    UseThumbBacktraces = true;
+  if (isTargetDarwin())
     IsR9Reserved = true;
-  }
 }