How do I get the standard default.prop file back on the root file system?
I'll just quote Elluel on XDA:


  Don't bother messing with reverting everything just to update, it is
  extremely unlikely to work since as of Lollipop Google has changed the
  OTA method to check the sum of the entire partition rather than only
  what it cares about.
  
  Download the 5.1 factory images, and flash system.img, boot.img,
  cache.img, radio.img and bootloader.img (don't flash userdata.img
  unless you want to wipe all your data), then boot into a custom
  recovery and flash the latest SuperSU
  (http://download.chainfire.eu/supersu).


It doesn't answer the question here directly, but it ultimately solved what I was trying to achieve.