Bug 901756 - Fix a few nits in nsCycleCollector.cpp r=khuey

Remove an unused forward declaration, and an unused field

xpcom/base/nsCycleCollector.cpp

@@ -909,8 +909,6 @@ class nsCycleCollectorRunner;
 // Top level structure for the cycle collector.
 ////////////////////////////////////////////////////////////////////////
 
-class AsyncFreeSnowWhite;
-
 class nsCycleCollector
 {
     friend class GCGraphBuilder;
@@ -2236,7 +2234,6 @@ public:
                            bool aAsyncSnowWhiteFreeing,
                            CC_ForgetSkippableCallback aCb)
         : SnowWhiteKiller(aAsyncSnowWhiteFreeing ? 0 : aMaxCount),
-          mCollector(aCollector),
           mRemoveChildlessNodes(aRemoveChildlessNodes),
           mAsyncSnowWhiteFreeing(aAsyncSnowWhiteFreeing),
           mDispatchedDeferredDeletion(false),
@@ -2280,7 +2277,6 @@ public:
     }
 
 private:
-    nsCycleCollector* mCollector;
     bool mRemoveChildlessNodes;
     bool mAsyncSnowWhiteFreeing;
     bool mDispatchedDeferredDeletion;