Remove the old ARM code generator from ART's Optimizing compiler.

The AArch32 VIXL-based code generator has been the default
ARM code generator in ART for some time now. The old ARM
code generator does not compile anymore; retiring it.

Test: test.py
Bug: 63316036
Change-Id: Iab8fbc4ac73eac2c1a809cd7b22fec6b619755db
diff --git a/compiler/optimizing/scheduler_arm.h b/compiler/optimizing/scheduler_arm.h
index 897e97d..a9f2295 100644
--- a/compiler/optimizing/scheduler_arm.h
+++ b/compiler/optimizing/scheduler_arm.h
@@ -17,20 +17,13 @@
 #ifndef ART_COMPILER_OPTIMIZING_SCHEDULER_ARM_H_
 #define ART_COMPILER_OPTIMIZING_SCHEDULER_ARM_H_
 
-#ifdef ART_USE_OLD_ARM_BACKEND
-#include "code_generator_arm.h"
-#else
 #include "code_generator_arm_vixl.h"
-#endif
 #include "scheduler.h"
 
 namespace art {
 namespace arm {
-#ifdef ART_USE_OLD_ARM_BACKEND
-typedef CodeGeneratorARM CodeGeneratorARMType;
-#else
+// TODO: Replace CodeGeneratorARMType with CodeGeneratorARMVIXL everywhere?
 typedef CodeGeneratorARMVIXL CodeGeneratorARMType;
-#endif
 
 // AArch32 instruction latencies.
 // We currently assume that all ARM CPUs share the same instruction latency list.