Revert "[FIXED JENKINS-20108]"

This reverts commit 68eb26633b1c042d90a426efac7b6e60dd4b2ede.
[FIXED JENKINS-22769] ItemListener callbacks should run as SYSTEM since they sometimes do ACL-checked calls.
(cherry picked from commit c04cdcd9f717ddcd3e8c9dbe86cb353c14ae511e)

Conflicts:
	changelog.html
[JENKINS-21224] Defend against exceptions in listeners, for now just for ItemListener.
java.lang.RuntimeException: Unable to copy /…/jobs/…/config.xml
	at hudson.plugins.jobConfigHistory.FileHistoryDao.createNewHistoryEntryAndCopyConfig(FileHistoryDao.java:252)
	at hudson.plugins.jobConfigHistory.FileHistoryDao.createNewItem(FileHistoryDao.java:238)
	at hudson.plugins.jobConfigHistory.JobConfigHistoryJobListener.onCreated(JobConfigHistoryJobListener.java:31)
	at hudson.model.listeners.ItemListener.fireOnCreated(ItemListener.java:161)
	at jenkins.model.Jenkins.putItem(Jenkins.java:2483)
	at …
Caused by: java.io.FileNotFoundException: /…/jobs/…/config.xml (No such file or directory)
	at java.io.FileInputStream.open(Native Method)
	at java.io.FileInputStream.<init>(FileInputStream.java:120)
	at hudson.plugins.jobConfigHistory.FileHistoryDao.copyConfigFile(FileHistoryDao.java:178)
	at hudson.plugins.jobConfigHistory.FileHistoryDao.createNewHistoryEntryAndCopyConfig(FileHistoryDao.java:250)
	... 108 more
(cherry picked from commit 6825121f06d6427304fd5149531216ecae6fdf93)
[FIXED JENKINS-25400] Rework fix of JENKINS-22769 (c04cdcd) to put the burden on each listener to impersonate ACL.SYSTEM if it needs to.
(cherry picked from commit a6a3d5e1660735edc18d331500f7ce9850fbc724)

Conflicts:
	changelog.html