How do I get rid of this power assertion from a dead process?
I had the same issue: a PreventUserIdleDisplaySleep assertion set by a process that was no longer active. Killing the powerd process (to be restarted by launchd) cleared the assertion without logging out or rebooting.

$ ps -e | grep powerd
63 ??         0:00.05 /System/Library/CoreServices/powerd.bundle/powerd
$ sudo kill 63


I'm not sure about any undesirable side effects might occur when killing powerd, however.