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_x86_64.h b/compiler/optimizing/code_generator_x86_64.h
index 57ef83f..eb082a0 100644
--- a/compiler/optimizing/code_generator_x86_64.h
+++ b/compiler/optimizing/code_generator_x86_64.h
@@ -253,12 +253,12 @@
   //
   //   root <- *address
   //
-  // while honoring read barriers (if any).
+  // while honoring read barriers if `requires_read_barrier` is true.
   void GenerateGcRootFieldLoad(HInstruction* instruction,
                                Location root,
                                const Address& address,
-                               Label* fixup_label = nullptr,
-                               bool requires_read_barrier = kEmitCompilerReadBarrier);
+                               Label* fixup_label,
+                               bool requires_read_barrier);
 
   void PushOntoFPStack(Location source, uint32_t temp_offset,
                        uint32_t stack_adjustment, bool is_float);