Remove useless class AbstractRepositoryManager

ItemRepositoryRegistry doesn't use AbstractRepositoryManager.
ItemRepository only forwards the manager to ItemRepositoryRegistry.
Therefore, these two classes don't need AbstractRepositoryManager.
Since AbstractRepositoryManager is not useful anywhere else either,
remove the entire class. 5128ac8d made
this abstract class useless by commenting out a single call to its
single member function:
     ~RepositoryManager() {
-      deleteRepository();
+      //Don't do this, we don't need it, and it may lead to crashes
+//       deleteRepository();
     }

No class inherits RepositoryManager, so it doesn't need a virtual
destructor.