Description: If an entry contains a to-be-encrypted attribute,
id2entry_add_ext handles a copy of the entry in which the attribute
is encrypted, then the copy is being stored in the backend db.
On the other hand, an entrydn attribute is supposed to remove from
the entry before storing the entry in the db. There was a bug
there: the removal was done on the original entry, but not on the
encrypted copy. This patch correctly removes the entrydn attribute
from the encrypted entry to be stored in the db.