Revert "Hard delete backup stack after successful update."

This reverts commit 7cf6e52cacd2382b44b9255e9a5ed13271841261.

This change appears to be the cause of many Tempest test breakages.  Prior
to this a new raw_template entry was created in the database after an
update, and the pointer to the new raw_template and the stack status were
changed atomically. Now those two pieces of data are being updated
separately... it seems like it is possible for the database to return the
new version of the stack on one call followed by the old version of the
template on another call. Tempest is observing the stack status change to
UPDATE_COMPLETE but then still getting data from the old template when
doing a resource list immediately after.

Change-Id: Ieea739bc45dcc365ccb6f09d0e848e310d327134
Partial-Bug: #1370865