ART: Remove some unnecessary mutator lock annotations.

The StackReference<> pointer held by a Handle<> can be used
without holding the mutator lock. We already do that when
we copy Handle<>s around. Only accessing the actual content
of the pointed-to StackReference<> needs to be done while
holding the mutator lock.

Change-Id: I5f93bd7e277383192f1f16dff6883ecb26387414
diff --git a/runtime/handle.h b/runtime/handle.h
index 5b3bb60..5df010b 100644
--- a/runtime/handle.h
+++ b/runtime/handle.h
@@ -70,8 +70,7 @@
     return reinterpret_cast<jobject>(reference_);
   }
 
-  ALWAYS_INLINE StackReference<mirror::Object>* GetReference()
-      SHARED_REQUIRES(Locks::mutator_lock_) {
+  ALWAYS_INLINE StackReference<mirror::Object>* GetReference() {
     return reference_;
   }