Revert "DiskWriteThreadPool's destructor is virtual for testing purpose"

This reverts commit ea0d44b7.

Reverting the previous commit as Steve wants to handle those changes, as part of a bigger reshuffling.

castor/tape/tapeserver/daemon/DiskWriteThreadPool.hpp

@@ -63,7 +63,7 @@ public:
    * Destructor: we suppose the threads are no running (waitThreads() should
    * be called befor destruction unless the threads were not started.
    */
-  virtual ~DiskWriteThreadPool();
+  ~DiskWriteThreadPool();
   
   /**
    * Starts the thread created at construction time.