py3: LDAPClient: remove __del__ method
    
    in py3 we are receiving ugly tracebacks due __del__ method that
    should be used very carefully or never. Due tracebacks, this doesn't
    work and context manager should be used for reliable connection
    termination.
    
    Exception ignored in: <bound method LDAPClient.__del__ of ipaserver.plugins.ldap2.ldap2()>
    Traceback (most recent call last):
      File "/usr/lib/python3.5/site-packages/ipapython/ipaldap.py", line 1057, in __del__
      File "/usr/lib/python3.5/site-packages/ipaserver/plugins/ldap2.py", line 123, in close
      File "/usr/lib/python3.5/site-packages/ipalib/backend.py", line 94, in isconnected
    NameError: name 'hasattr' is not defined
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>