[JENKINS-29936] when removing an item use ACL.SYTEM.

The OldDataMonitor should be using ACL.system not the ACL of the calling
thread - this also avoids the deadlock when an authorization strategy is
being saved (locking the auth strategy) which will call into the ODM at
the same point the ODM is being called an a Run has been saved (which will
cause a lookup of the job which will do a permissions check).