Clean up read barrier related comments in Optimizing.

Bug: 12687968
Change-Id: Idf2e371e01e10d9d32c95b150735e2c96244232e
diff --git a/compiler/optimizing/code_generator_x86.cc b/compiler/optimizing/code_generator_x86.cc
index 53e33bf..6a9177d 100644
--- a/compiler/optimizing/code_generator_x86.cc
+++ b/compiler/optimizing/code_generator_x86.cc
@@ -4854,7 +4854,7 @@
     // Temporary registers for the write barrier.
     locations->AddTemp(Location::RequiresRegister());  // Possibly used for ref. poisoning too.
     // Ensure the card is in a byte register.
-    locations->AddTemp(Location::RegisterLocation(ECX));  // Possibly used for read barrier too.
+    locations->AddTemp(Location::RegisterLocation(ECX));
   }
 }