ARM: Purge Arm32Assembler.

Use Thumb2Assembler always. This originated from finding out that
the JNI tests are run using the Arm32Assembler however in real
world Thumb2Assembler is used for JNI. Therefore Arm32Assembler
code is dead except its own tests and the illegitimate use in
JNI tests.

Change-Id: I9ca6b83582bf97149a46690518ccb9312b1a3b68
diff --git a/compiler/optimizing/optimizing_compiler.cc b/compiler/optimizing/optimizing_compiler.cc
index a1da20b..698b0b6 100644
--- a/compiler/optimizing/optimizing_compiler.cc
+++ b/compiler/optimizing/optimizing_compiler.cc
@@ -833,9 +833,7 @@
 
   // Always use the Thumb-2 assembler: some runtime functionality
   // (like implicit stack overflow checks) assume Thumb-2.
-  if (instruction_set == kArm) {
-    instruction_set = kThumb2;
-  }
+  DCHECK_NE(instruction_set, kArm);
 
   // Do not attempt to compile on architectures we do not support.
   if (!IsInstructionSetSupported(instruction_set)) {