Cache: Remove dangling doWriteback declaration

This patch removes the declaration of doWriteback as there is no
implementation for this member function.

src/mem/cache/cache.hh

@@ -340,12 +340,6 @@ class Cache : public BaseCache
      */
     void markInService(MSHR *mshr, PacketPtr pkt = 0);
 
-    /**
-     * Perform the given writeback request.
-     * @param pkt The writeback request.
-     */
-    void doWriteback(PacketPtr pkt);
-
     /**
      * Return whether there are any outstanding misses.
      */