Revert "Set nova.migration_interval to 3 seconds"

No need after merging [1].

[1] https://review.opendev.org/c/openstack/watcher/+/971007

Depends-On: https://review.opendev.org/c/openstack/watcher/+/971007

This reverts commit 1636bed.

Author: amoralej

templates/00-default.conf

@@ -142,6 +142,3 @@ period = 900
 
 [watcher_cluster_data_model_collectors.storage]
 period = 900
-
-[nova]
-migration_interval = 3