Fixing compile issue on Mac (#6579)

Problem
Compiler complains about unused variable

Summary of Changes
- Updated the CHIPPersistentStorageDelegateBridge.h file

Test
- Used the gn_build.sh to verify the building is successful

src/darwin/Framework/CHIP/CHIPPersistentStorageDelegateBridge.h

@@ -38,7 +38,6 @@ class CHIPPersistentStorageDelegateBridge : public chip::PersistentStorageDelega
 
 private:
     id<CHIPPersistentStorageDelegate> mDelegate;
-    dispatch_queue_t mQueue;
 
     NSUserDefaults * mDefaultPersistentStorage;
     dispatch_queue_t mWorkQueue;