Remove unused DEFAULT_DOMAIN variable

The keystone.identity.backends.ldap.DEFAULT_DOMAIN
variable wasn't used.

The keystone.assignment.backends.ldap.DEFAULT_DOMAIN
was used, but should not have been, because we've
got a keystone.assignment.core.DEFAULT_DOMAIN.

Change-Id: I0f4bb0bd953a9ced8c3dfc46ea44ff0e377db082
Merge "Remove unused DEFAULT_DOMAIN variable"