sysupdated: Make sure targets we skip are skipped

sysupdated: Make sure targets we skip are skipped

We'd log that we're skipping the target, but it would never actually get
removed from the manager's list. Thus, we'd advertise targets that don't
actually exist to clients.

In the original version of the sysupdated PR, this was handled by
removing the target from the manager's list in target_free, and using a
_cleanup_ attribute to free the target when skipping. However, this
changed at some point during review. So, this commit takes the
alternative approach