ipautil: remove get_domain_name()

get_domain_name() and related code depends on ipaplatform.

Replace all uses of get_domain_name() with api.env.domain and remove
get_domain_name() and all of the related code.

https://fedorahosted.org/freeipa/ticket/6474

Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>

ipapython/ipautil.py

@@ -50,7 +50,6 @@ from six.moves import input
 from six.moves import urllib
 
 from ipapython.ipa_log_manager import root_logger
-from ipapython import config
 from ipaplatform.paths import paths
 from ipapython.dn import DN
 
@@ -63,16 +62,6 @@ KRB5KDC_ERR_SVC_UNAVAILABLE = 2529638941 # A service is not available that is
                                          # required to process the request
 
 
-def get_domain_name():
-    try:
-        config.init_config()
-        domain_name = config.config.get_domain()
-    except Exception:
-        return None
-
-    return domain_name
-
-
 class UnsafeIPAddress(netaddr.IPAddress):
     """Any valid IP address with or without netmask."""