Remove precheck for nova_enable_rolling_upgrade

This variable was removed in the Train cycle, and a precheck added for
its use. This precheck can now be removed.

Change-Id: I6d9f0b577631ff9443deecf8ef9d94ca217674c5

ansible/roles/nova/tasks/precheck.yml

@@ -33,11 +33,3 @@
     - container_facts['nova_api'] is not defined
     - inventory_hostname in groups[nova_api.group]
     - nova_api.enabled | bool
-
-# TODO(mgoddard): Remove this task in the Ussuri cycle.
-- name: Check that legacy upgrade is not enabled
-  fail:
-    msg: >
-      Legacy upgrade support has been removed. 'nova_enable_rolling_upgrade'
-      should no longer be set.
-  when: not nova_enable_rolling_upgrade | default(true) | bool