Don't delete sys_meta on instance delete

Unfortunately, we require to access instance_system_metadata to get
data (specifically at least instance_type data) for instances that have
been deleted.

There's 2 cases where this is true:

1) nova-api supports showing deleting instances when you specify a
'changes-since' param.
2) The _usage_audit_log periodic task pulls all instances during the
audit period, which includes instances that have been deleted during
that period.

This reverts commit 17bca438954638d74035d560e826a26a532b3ea3, which was
attempting to fix a bug where we leave instance_system_metadata entries
undeleted from the DB.  There's not an easy way to query for the deleted
sys_meta data in an efficient manner, and leaving the entries undeleted is
the lesser of the 2 evils for now.

Fixes bug 1185190

Change-Id: I898f0546c49126dcc56a4237120082f95dc82304
(cherry picked from commit 4885aa28706a1858f4fc51a0d2c661eec05139c0)