remove n-sch from the watch list

we've seen no n-sch errors in passing gate runs in the last week,
I consider this sufficient proof that we're clean on n-sch for now.

Part of bp:clean-logs

Change-Id: Ife0e0602767e155fc645e07546bed36c2c060a20
Merge "remove n-sch from the watch list"
Merge branch 'master' of https://github.com/openstack/tempest