rendercache/decodercache: moved qmutex that was only used by decodercache into

decodercache's derived class

app/render/backend/rendercache.h

@@ -18,17 +18,9 @@ public:
 
   bool Has(K key) const {return values_.contains(key);}
 
-  void Lock() { lock_.lock(); }
-
-  void Unlock() { lock_.unlock(); }
-
-  QMutex* lock() { return &lock_; }
-
 private:
   QHash<K, V> values_;
 
-  QMutex lock_;
-
 };
 
 #endif // RENDERCACHE_H