Remove default argument values in GenerateGcRootFieldLoad.

These values were never or rarely used.

Test: mmma art (with and without `ART_USE_READ_BARRIER=true`)
Bug: 12687968
Bug: 29516974
Change-Id: I5d15140ce501bf50d7a87871b1e492cee54913db
diff --git a/compiler/optimizing/code_generator_arm_vixl.h b/compiler/optimizing/code_generator_arm_vixl.h
index 1cd6184..b0fa038 100644
--- a/compiler/optimizing/code_generator_arm_vixl.h
+++ b/compiler/optimizing/code_generator_arm_vixl.h
@@ -339,7 +339,7 @@
                                Location root,
                                vixl::aarch32::Register obj,
                                uint32_t offset,
-                               bool requires_read_barrier = kEmitCompilerReadBarrier);
+                               bool requires_read_barrier);
   void GenerateTestAndBranch(HInstruction* instruction,
                              size_t condition_input_index,
                              vixl::aarch32::Label* true_target,