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_arm64.h b/compiler/optimizing/code_generator_arm64.h
index eb28ecb..7de84be 100644
--- a/compiler/optimizing/code_generator_arm64.h
+++ b/compiler/optimizing/code_generator_arm64.h
@@ -289,13 +289,13 @@
   //
   //   root <- *(obj + offset)
   //
-  // while honoring read barriers (if any).
+  // while honoring read barriers if `requires_read_barrier` is true.
   void GenerateGcRootFieldLoad(HInstruction* instruction,
                                Location root,
                                vixl::aarch64::Register obj,
                                uint32_t offset,
-                               vixl::aarch64::Label* fixup_label = nullptr,
-                               bool requires_read_barrier = kEmitCompilerReadBarrier);
+                               vixl::aarch64::Label* fixup_label,
+                               bool requires_read_barrier);
 
   // Generate a floating-point comparison.
   void GenerateFcmp(HInstruction* instruction);