Use joined version of db.api calls

Replace db.instance_get_active_by_window with
db.instance_get_active_by_window_joined

Remove db.instance_get_active_by_window as it is now unused

When moving over to nova.db.api not returning sqlalchemy
objects non-joined db calls (lazy loaded) won't work.

Partially implements bp db-api-cleanup

Change-Id: Iec647792a049d53862a45f95accd7f46e483aa17
Merge "Use joined version of db.api calls"