Remove unused admin_hosts_client

admin_hosts_client is not used in LiveBlockMigrationNegativeTestJSON,
so this is to remove it for code clean.

Change-Id: Iae2db538ae6711104da7048e3de61cfa4c129a93
diff --git a/tempest/api/compute/test_live_block_migration_negative.py b/tempest/api/compute/test_live_block_migration_negative.py
index bd2b185..f072b81 100644
--- a/tempest/api/compute/test_live_block_migration_negative.py
+++ b/tempest/api/compute/test_live_block_migration_negative.py
@@ -33,7 +33,6 @@
     @classmethod
     def setup_clients(cls):
         super(LiveBlockMigrationNegativeTestJSON, cls).setup_clients()
-        cls.admin_hosts_client = cls.os_adm.hosts_client
         cls.admin_servers_client = cls.os_adm.servers_client
 
     def _migrate_server_to(self, server_id, dest_host):