openchange/MAPIStoreFolder: Add private selector for actual folder delete action

Usually, it is a bad idea for an object to call its own public methods
(just like in this case). Thus separating impelementation for deleteFolder:
would allow MAPIStoreFolder to call only private implementation when needed

Signed-off-by: Kamen Mazdrashki <kmazdrashki@zentyal.com>
Merge pull request #13 from Zentyal/kamenim/fix-crash-delete-folder

Fix crashes during Move/Delete folder