Remove redundant db.volume_update() in volume manager create_volume()

Remove redundant db.volume_update() in volume manager create_volume()

Since scheduler has already updated db to set the 'host' field and
recorded 'scheduled_at' time, it's not necessary to do db.volume_update()
again in volume manager create_volume() method.  Also let volume api
to update db when creating volume from snapshot while snapshot_same_host
is true, thus the behavior is consistent (updating db when placement
decision is made).

Change-Id: I43aaee8e2a5d0ceadb98bd608cc147467488349e