How to remove undeletable login item
Since it is set for all users it should be in the

/System/Library/StartupItems


or in the /System/Library/LaunchDaemons 

Correction thanks to @maximus that would be in the /Library/LaunchAgents

Copy one of the above and paste it in the Finders "Go to Folder"

Alternatively hover the mouse pointer over it (while it is highlighted) and there should be a pop up with the location of that file.