ERP5Type: Include OFSFolder2._cleanup method

  This aims to add compatibility with BTreeFolder2 API, even it is not
  required. Since some checkConsistency may call self._cleanup() regardless
  expecting that the folder is a [H]BTreeFolder2 always.

  This was detected when a post upgrade constrant was included to
  portal_categories

product/ERP5Type/Core/Folder.py

@@ -667,6 +667,12 @@ class OFSFolder2(OFSFolder):
     except AttributeError as exc:
       raise KeyError(exc.args)
 
+  def _cleanup(self):
+    # Keep compatibility with BTreeFolder2 API, despite it is not reaquired.
+    # This api is required by the [check,fix]Consistency implementation.
+    LOG("OFSFolder2._cleanup", WARNING, "This folder class do not implement _cleanup, skip.")
+    return True
+
 OFS_HANDLER = 0
 BTREE_HANDLER = 1
 HBTREE_HANDLER = 2