Remove unused-but-set variable from objects.cc.

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index fb09978d5770a176b42ec54e0620cd6e205868aa..2f2d2ad8846fc23041e42d905a858751843f7ffb 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -7562,12 +7562,10 @@ MaybeObject* JSObject::OptimizeAsPrototype() {
 
   // Make sure prototypes are fast objects and their maps have the bit set
   // so they remain fast.
-  Map* proto_map = map();
   if (!HasFastProperties()) {
     MaybeObject* new_proto = TransformToFastProperties(0);
     if (new_proto->IsFailure()) return new_proto;
     ASSERT(new_proto == this);
-    proto_map = map();
   }
   return this;
 }