Remove the old ARM assemblers from ART.

Now that the old ARM code generator for ART's Optimizing
compiler is gone, these assemblers no longer have users;
retiring them.

Test: test.py
Bug: 63316036
Change-Id: Iaea42432a9e0d3288b71615f85c58846c0336944
diff --git a/compiler/optimizing/instruction_simplifier_arm.cc b/compiler/optimizing/instruction_simplifier_arm.cc
index 3fc7c50..fe22595 100644
--- a/compiler/optimizing/instruction_simplifier_arm.cc
+++ b/compiler/optimizing/instruction_simplifier_arm.cc
@@ -147,8 +147,8 @@
   Primitive::Type type = instruction->GetType();
 
   // TODO: Implement reading (length + compression) for String compression feature from
-  // negative offset (count_offset - data_offset). Thumb2Assembler does not support T4
-  // encoding of "LDR (immediate)" at the moment.
+  // negative offset (count_offset - data_offset). Thumb2Assembler (now removed) did
+  // not support T4 encoding of "LDR (immediate)", but ArmVIXLMacroAssembler might.
   // Don't move array pointer if it is charAt because we need to take the count first.
   if (mirror::kUseStringCompression && instruction->IsStringCharAt()) {
     return;