Delete InstanceSystemMetadata on instance deletion

Thanks to no-db-compute only pre-delete instance data is used in
notifications. So now we can go back to deleting InstanceSystemMetadata
when an instance is deleted.

Fixes bug 1153827

Change-Id: Ic66b998cde2a15a24f302f08c34753a8b57da73d
(cherry picked from commit 17bca438954638d74035d560e826a26a532b3ea3)
Merge "Delete InstanceSystemMetadata on instance deletion" into stable/grizzly