ldap2.can_read: fix py3 compatibility
    
    As with commit b37d18288d, can_read() method does not need to decode
    a string in Python 3. can_read() wasn't used anywhere in the code,
    apparently.
    
    Related: https://pagure.io/freeipa/issue/7953
    Reviewed-By: Christian Heimes <cheimes@redhat.com>