Remove _migrate_server_to's return value

The return value of _migrate_server_to is never used, and
it seems will not be used in the future. This is to remove
it for the code clean.

Change-Id: I81184ed26b3c72590a749540b3ab6f79db8eb571
diff --git a/tempest/api/compute/admin/test_live_migration.py b/tempest/api/compute/admin/test_live_migration.py
index 72d5b18..ff84945 100644
--- a/tempest/api/compute/admin/test_live_migration.py
+++ b/tempest/api/compute/admin/test_live_migration.py
@@ -72,10 +72,9 @@
             block_migration = (CONF.compute_feature_enabled.
                                block_migration_for_live_migration and
                                not volume_backed)
-        body = self.admin_servers_client.live_migrate_server(
+        self.admin_servers_client.live_migrate_server(
             server_id, host=dest_host, block_migration=block_migration,
             **kwargs)
-        return body
 
     def _get_host_other_than(self, host):
         for target_host in self._get_compute_hostnames():
diff --git a/tempest/api/compute/test_live_block_migration_negative.py b/tempest/api/compute/test_live_block_migration_negative.py
index f072b81..7853962 100644
--- a/tempest/api/compute/test_live_block_migration_negative.py
+++ b/tempest/api/compute/test_live_block_migration_negative.py
@@ -37,10 +37,9 @@
 
     def _migrate_server_to(self, server_id, dest_host):
         bmflm = CONF.compute_feature_enabled.block_migration_for_live_migration
-        body = self.admin_servers_client.live_migrate_server(
+        self.admin_servers_client.live_migrate_server(
             server_id, host=dest_host, block_migration=bmflm,
             disk_over_commit=False)
-        return body
 
     @test.attr(type=['negative'])
     @test.idempotent_id('7fb7856e-ae92-44c9-861a-af62d7830bcb')